I can't agree with simply focusing on all your teams equally. Focus on your helper team first, THEN focus on whatever you want. Unless you have prior arrangements, people you want as helpers aren't going to want you as an ally unless you can help them as well. Two really strong teams (one of them a helper) are going to be enough to get you through a large majority of the maps. For those maps that require a more balanced approach you can split up your helper team to shore up the weaker teams.

Regarding 3* girls specifically, they're not BAD, but their ability to equip four items isn't really a selling point if the higher star girls beat their stats with one equip. Level all of the ones in your teams some, but save evolution for your favorites (doesn't really matter too much which ones). That way you won't have wasted leveling/evolution on girls who will be replaced every two weeks with event girls. I've still got two evolved (but not maxed) 3* girls hanging onto my weakest team by their fingernails (and several 4*s who never did make any teams, so as I said, they're not BAD), but after I get the next event girl at least one of them will be dumped.

To be honest, Flower Knight Girl isn't all that difficult and the phase where you have to worry about stuff like this doesn't last too long. As long as you keep playing regularly, soon enough all your teams will be strong enough without really having to focus your efforts anywhere.

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.

Probably a good but not the BEST idea, but sort your units in priority (High-Low) and level top-left to bottom right. I've noticed that units with higher rarities PROBABLY have better stats than those in lower rarities. Hit that normal gacha every day and see if you can get four-stars, and I'd say to throw out the three-stars that don't/wouldn't make it into the 3rd or even 4th squad.