ideally you should save them for a girl you want, believing in the chance of getting her, unless nutaku puts a really tempting limited gacha (like the limited 2 slyce type gacha) and you feel that you have more chances of getting something good there

Personally I only evolved 2 3* before I got 5/4s to fill out my team. My advice is to not evolve them until you have evolved and lvled your 5/4. Remember you get a new 5* every 2 weeks (events) and gacha seeds should eventually fill out your team with 4*. Also if you do decide to evolve them make sure its to fill in the weak spots (such as magic or slice) of your team since you cant really choose which 5/4* you get.