If you can't implement an enjoyable, fairer  BASIC CLASSIC VANIILLA arena system , don't go creating a  "new, better but not really" 3v3 arena.

I thought this would be common sense which most humans learn in early childhood. 

Whats worse is that the basic arena system has been done and done again in hundreds of similar games. It's one of the basic features that gacha gamers have come to expect. Surprisingly it was not possible by this company. Likely this was done on purpose as just another method of heavy handing the players to spend money. Every annoying, unfair, rigged feature in this game were all done for the same reason. 

If they actually implement this in a fresh fun way it would soften the blow a lot. However, this 3v3 arena is uninspired, unoriginal, vanilla and a totally obvious ploy to make players have to gear up more chars, which means spend more time farming OR.... pay up

Geez, at least put some thought into it and create a fun system that encourages play rather than feeling like yet another daily chore that is exactly like the other chores.

If you really can't come up with something, why not do what you did at the start? Just copy/paste from another game.