I don’t think this is what Sin is asking for. Fact is a 3v3 game is neither fun nor conducive to filling a server. If you’re lucky people will derp in warmup or play a crappy game until others join, however what is likely to happen is that people just leave the server. Building on that seeing a server with 5-6 people on it is going to passed over by people, why join something that’s likely to waste your time.
So yes, an active game with a large userbase will perhaps get over this by placing people via quickgame/matchmaking into such matches quickly. However if the userbase isn’t large enough you get a lot of atrophy as players find getting into a fun match more of a chore.
This brings us back to two things. One is having options to join a server in ways that ensure a minimum of people are up for playing, see my thread on that. The other is giving those people on a server something to do which is fun while you wait for it to fill. For that second part I could suggest warmup type games, deathmatch, catch the irradiated rabbit, gun game. Yet the problem there is people would be joining for objective and may have no interest in those sorts of distractions, back to square one.
So that leaves Sin’s suggestion, make a mini-objective type game that does work…
… I’ve said enough. 