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.