Why won't people play a 3v3v3 with Diplomacy? - So it becomes a 4v5, where you pick your own team.  Or a unlocked 2v2v2 becoming 3v3 of you own choice?  Rather than time consuming PUGs, which never seems to get started.  In PUGs, Nobody ever seems to want to be a captain, and then it also takes forever to pick teams.  Invariably someone will bail out because they are unhappy with their team.  Or maybe someone drops, meaning you have redo the whole proccess again.  AND smurfs ruin the whole proccess anyway!

If you start with 3 unlocked teams (not a completely unlocked FFA), you start with other teammates with a 10 relations (rather than 0).  Then there is an incentive to keep these teammates to easily form pacts with, as well as benefit from the bonuses that envoys bring.  Also, the investment that you make in them helps mainatain loyalty.  Granted, you also prefer the ajoining players to be teammates, and this may persuade you change teams, but ultimately a team of victors will emerge.

At the start of the game, set the size of victor's team in the game's settings.  Any winning team with a number of players smaller or equal to that number will be the victors.  For a 3v3v3 set it at 5 players, for 2v2v2 set it at 3 players.