I've played a lot of 3v3 !ulti games and it's been quite a lot of fun for me. The concept of ultimate builder is pretty cool and sometimes you would get insane combinations which are extremely fun.
Overall balance is obviously good but with the only one exception: the Artillery.
Don't get me wrong, I don't consider it overpowered in 2v2 although it's pretty strong and you really have to delay the game until you're able to build enough of them which could be hard due to Human units falling off in late game.
But there's few things to consider with having Artillery in a 3v3 !ulti mode:
You have random set of units which already could be good by themselves, instead of Human units which are in general a bit weaker and give good room for winning the game before Artillery fucks up your castle
You have 2 people delaying the game for your Artillery instead of 1 in 2v2, which means bigger percentage of recources is used for actual army
At the same time, it also means that your Artillery becomes more useful earlier because of bigger amount of buildings is done (3 people making them instead of 2) and thus it is easier to hit them
I think it is balanced enough in 2v2 mode, but in 3v3 it should be removed completely as the aforesaid advantages it has are rather unfair. I've had some cool and epic games with or against it in 2v2, while in 3v3 it's always easy win for the team that has it (thank god I haven't had a game in which 2 people on the same team would have it xD).