the reasoning against 3v3 is that there are only 27 teams in the 2v2 ladder, and the 2v2 template, while not as great as the 3v3 one, is a built in template and stuff, so a 3v3 ladder would have about 5-10, maybe 15 teams, and there wouldnt be any compettion
Make it unlockable from level 50-60 or higher to accept a 3vs3 invitation to play on the ladder.
Mind though that this does NOT mean you can start your own group of players for the 3vs3 ladder settings but can only join if invited by actual members.
Seems like your problem can easily be solved....
I know this also makes it easy for some to join with their alt's so we'll limit the invitations to non members to a maximum of 1 so as not to have Gnuff or Pulsey with three accounts