There really aren't any 3v3s out there; most TOs usually do 4v4s (or "squads" as they're mostly referred to). However, I can't really make an argument for 4v4 over 3v3 other than 4v4s are entertaining as hell.
Here's some cool 4v4 sets:
https://www.youtube.com/watch?v=ivbJj2kPF28 - KTAR XI - Chansey's Chaperones vs Deadly Alliance - SSB4 Squad Grand Finals - Smash for Wii U
iirc, one of the first 4v4s streamed. This tourney was a few days after the Wii U release, so the stagelist was very early, since they had Temple as a counterpick (which I believe is not the case anymore?). A great example of "roles" in 4v4s, such as the rushdown, the stock tank, the projectile wall, and the wildcard. Also, did I mention Temple?
https://www.youtube.com/watch?v=qXeGEZkwh3c - S@X - Dynasty (Blue) Vs. #WeDemBoys (Red) SSB4 Squads Winners Quarters - Smash 4v4
Just a fun set. Though, Kongo Jungle 64 is played here. However, it seems to be somewhat small for 4v4s? But nonetheless, I like that stage on 4v4. Also, it shows just how difficult it is to make comebacks in 3v1 situations.
https://www.youtube.com/watch?v=3ZWveQTKmkk - DBA2 Smash Wii U - Project Smash vs The Harlem Globetrotters - 4v4 Round Robin
Another fun set, but it shows another competitive implication of 4v4s - partners. Mostly noted with the Villager and Megaman duo, pocketing those fully charged fsmashes. It also shows just how important teamwork and communication is in this game mode. Also, Battlefield was apparently okay here, but it's also pretty small. But hey, smaller stages makes it even more fun.
Overall, I
love 4v4s. I want to see it at the next Apex. I want to see more 4v4s in general. And with the expanded stage list with removed hazards from the recent update, there could be so much potential for a more MOBA based competitive Smash mode, what with the teamwork and roles you can take from it.
But right now, we need a universal stage list.