I would only use a captain if the responsibility actually means something useful.
If “captain” just means wearing an armband and being told you are in charge, I do not think 3v3 needs it.
If you want to give children little responsibilities, you can do that without a title: one helps collect the spare ball, one checks the group is at the right pitch, one starts a team high five, whatever fits your environment.
Then everybody can experience responsibility without one child becoming the boss.
I would be especially careful if the captain starts deciding who takes restarts or telling teammates where to stand.
At U7, leadership is more interesting when it looks like helping others than when it looks like copying an adult team structure.
