3v3 is entirely untested and unplayable in this game (with exception in this very only mode). There is no reason to have it tbh - with exception of 10% who actually like it for easy defensive gac wins. Some GLs are unbeatable entirely by other gl due to lack of teams and the only fun in 3v3 is to put a brick defense and lol at the person simply failing a GL counter. After that its over and you won.
So it caters a minority to have untested and unbalanced stuff - can be fun for some. But if you could choose 5v5 or 3v3 i would always click 5v5 - because those are teams actually and not a rng fest if 1 attack/ability block misses etc.