Rival Nations (MicrosoftXP, 2002-2004) is a well-established member of the 3v3v3 sub-genre of AoS maps. It has a couple of neat ideas that make it stand out from its peers, including the ability to control several heroes at once.
3v3v3 AoS maps have always been a popular choice among public players. They're easy to jump into, the small teams make it easy to bring a friend or two along, and they almost invariably have the familiar cast of Warcraft 3's default heroes available to choose from (sometimes supplemented with heroes of the developer's design, though usually they are low-effort ability swaps to pad out the map).
Conversely, many of the more dedicated AoS players avoided the 3v3v3 sub-genre due to a lack of original heroes, the difficulty of balancing private games, and the political rather than competitive nature of the gameplay.
Rival Nations lies on the upper end of complexity for 3v3v3 maps, due to having a number of features which demand a lot of attention and micro in order to play well.
The layout of Rival Nations is the familiar triangle seen in most 3v3v3 AoS maps, but with two parallel lanes along each outer edge of the triangle. The inner triangle is remarkably small compared to the amount of traffic that will traverse it at all stages of the game, making it a hotbed of gold and experience.
I am not a fan of the outer lanes being completely isolated from the inner lanes by thick walls of cliffs and trees. Yes, it means that if the blue team is particularly dangerous, you can hide out very safely on the red/yellow lane, but it also makes ganking and travelling around the map clunky and unnecessarily difficult.
One of the things I like on 3v3v3 maps is hanging out on a lane where everyone's your enemy to maximise farm at a risk, but that's not really possible here. Having the outer lanes split in two also seems like a stylistic choice rather than an practical one, since the split in the lanes makes gaining experience there even less favourable compared to the inner lanes. That said, as more hero units enter the map, having extra lane space available isn't all bad.
I do like that the base towers come in terraces of three, which makes area-of-effect abilities that damage buildings much more attractive, since you can hit the group at once. Contrast with most AoS maps where buildings are usually standalone and isolated. It's a subtle thing, but gives those abilities a bit more oomph. (Rival Nations uses the default cast of Warcraft heroes, so abilities that damage buildings are common.)
Another defensive feature is the Captain stationed in each town, a powerful unit that guards the main base. Timed right, a player using the Paladin hero can use its ultimate to resurrect a Captain under their own control, which is a nice trick.
I like the placement of fountains in each base, because once a team is losing one half of their base, they can still heal and recuperate in the other half, but there's no inner sanctuary to run to if both entrances are under attack.
Around the middle of each outer lane is a Trainer Hall. These buildings allow players to hire Trainers, a special kind of unit which can recruit specialised troops to augment your team's forces. But rather than directly produce units in RTS style, trainers must stand beside a friendly barracks, and will cause that barracks to periodically produce extra units of the appropriate type for its troop waves.
Teams may employ a maximum of 8 trainers. The neat part is that because trainers are units, you can move them from barracks to barracks depending on what your opponents are doing, which is the bigger threat, and what your team needs to accomplish.
Also worth noting is that because trainers are units, they are vulnerable to enemy heroes, and can potentially be killed by "trainer" raids or long-ranged harassment. Both of the above combined ensure that trainers are absolutely not a "buy and forget" mechanic.
The Trainer Hall also has a few other units for sale, including a powerful Golem, a catapult for taking out enemy towers, and a peasant which can build new defensive towers for your team, or gradually repair existing ones for a fee.
The location of the Halls discourages moving these units to the mid lane, but it's possible if they're routed through your own base.
Everything inside the trainer hall costs wood, which is a rather unusually implemented resource. Wood is granted for killing everything except heroes: contrast with most situations where designers add a resource that encourages hero killing. In Rival Nations there's a resource so you'll remember to kill everything else!
Wood is also required for troop upgrades, a standard selection of which are available in each team's base. It is always nice to see optional tower upgrades, but the rest were the usual "make troops better" that requires no thought on behalf of players.
With trainers in the game, upgrades felt redundant and a bit archaic. The upgrades do take some time to complete though, so unlike trainers, it's important to start investing in them early.
The concept of a "hero factory" is unfamiliar to most AoS players, but essentially it means that players can pick another hero. Not a repick though: in Rival Nations you can play with up to 7 heroes at once, and will almost certainly have at least a few by the end of the match.
Your first hero is free, and thereafter heroes cost an increasing sum of gold and wood to purchase. Players may pick any heroes they like in any order they like, though they are encouraged to follow the Warcraft 3 lore, as a special "racial bonus" is obtained upon grabbing all four heroes from the same race.
These "racial bonuses" take the form of an item which has strong passives, and can summon reinforcements from that race's usual forces. Each racial bonus also has a special one-time effect: for example, the Undead bonus sends invisible Shades to scout around the map, while the Night Elf bonus summons a number of tree-men to stand guard in your base.
Typical player behaviour is to grab many of one type of hero, such as those with disables or nukes that can be focused on one target to ensure a kill. Since the default Warcraft races are designed to not have overpowering combos like these, the racial bonuses actually increase diversity by giving players a reason to not always pick these combos.
Furthermore, the possibility of multiple racial bonuses discourages teams from massing many of the same hero.
Heroes are spawned with a Spirit of Preservation that will revive them on the spot after a short delay if they die. Once it's two charges are used, and the player dies without having bought more charges, they must either wait to revive, pay gold to revive early, or have the option of re-entering the hero select screen, and picking a new hero to start with from level 1. The option helps losing players to re-evaluate their approach.
Players also accumulate bounty as they play the game. Buying new heroes, killing enemy players or buildings, or levelling up will all add bounty. When any of a player's heroes die, a proportional amount of that bounty is subtracted, and given to the killer as gold: 100% if they only own 1 hero, 50% if they own two, etc.
This helps players figure out who to team up against and target, since bounties are listed on a leaderboard. The game also implemented a simple assist system, which was among of the first of its kind.
There are certainly more complicated AoS maps around than Rival Nations, but it has about the right amount of content for a 3v3v3 game. Its Trainers are a really nice way to handle shifting alliances and priorities. Since there's lots of ways to spend resources, there's a good amount of room for players to try different things, like maxing one hero with items first, or focusing on upgrades, or massing heroes, or getting racial bonuses early. Overall, a pretty nice package if you enjoy stock heroes and microing many of them at once. Download: Here