Unless you plan to throw real money at the game, you'll only get about 1 gold girl every 2 weeks. Which means you'll use some of your 3-star girls for quite a while. So you have to level them until you find a replacement, just to progress within the campaign. Focus on your gold girls, but feed the manyus for which you don't have a gold girl of matching color to your 4* and 3* girls.
My Sasanqua even reached lvl 60 evolved before I finally replaced her, and she's still my Pierce unit with sixth highest ATK, right next to lvl 60 Chocolate Cosmos, so 3* girls aren't really bad, they just lack a passive.