With the way he gets stronger, if he manages to end up with them 3 in bed, then after few "sessions" he will start to tire them out. They will need to live with that or get more waifus to bed at the same time, like the tsundere or the hero girl, and all other girls at school later when they find out how OP he can get.
All he need is shadow clone jutsu and he probably could bed girls in the whole planet at once after a short training.