Depends on your objectives, I suppose. For simply clearing maps and try to spawn SGs, having one high-powered team will get you farther than splitting your best units amongst the 4. Also, unless I'm mistaken, your friends' helper teams get scaled down (maybe up if they're lower level?) to your best girl so having at least 1 high level girl you've raised a lot should help. Many maps can be cleared (not 3-medal'd but just cleared) with just 1 team + helper, then you spawn SGs, get the manyus from there and raise your weaker girls. This is, if I'm not mistaken, the "suggested powerleveling" route, but like the above unregistered poster said, as long as you play regularly, the phase where your teams are weak enough you have to worry about being able to clear content won't last that long.

That said, in general, your 2/3*s are going to be replaced before too long, as you'll get event 5*s, 4*s here and there from normal gacha and if you spend the FGs you save up you might get a few good premium units as well, so as I said before, unless you're using 2/3*s with extremely high/low speed to adjust your teams' speeds to meet map requirements, I wouldn't worry about raising them too much (in particular the 2*s who even with 4 equip slots and skill level 5 get outclassed REAL QUICK by the 4/5*s, especially if you do well enough in events to get at least a 3 equip, skill level 4 event 5* every 2 weeks.