Do you ever ask one player to stay back in 3v3?

I’m with you — I would not solve that by appointing a child to stay back. Conceding because all three attacked is actually a useful picture for them. Next time you can ask, “when we all go to the ball, what is behind us?” and see whether one of them starts recognising it. That does not mean nobody can ever be deeper. One player may naturally hang back for a moment because they have seen the danger. The difference is that they chose it from the game rather than being given the job permanently. At U7 I want all three to experience scoring, stopping goals, dribbling, chasing back and helping teammates. If we create a centre-half at six because the adults dislike conceding into an empty goal, we are probably fixing the score before we fix the learning.