There's no reason to waste manyus and gold on something you're eventually going to merge anyways as your event golds pile up. 3 stars are completely fine in the transition from early to midgame, you don't even need to invest much in them. I only leveled 4 of mine past 30, only 1 to 40, the rest were kept in mid-high 20's and managed to hold out until I got more silvers. Unless you have some god-awful luck, 4 11 pulls should give you enough silvers to flesh out a solid team, and then you choose a few 3 stars to level past 30 and fill until you get some more pulls.