I used to keep one copy of every 3* knight. But eventually i started to wonder why i should bother to waste so much of my knight capacity just for some collector reason. Especially since it isn't rly hard to get them anew. So i sold every 3* knight who was below lvl 40 and I haven't regreted it since then. Now i sometimes add one of those 3* again to my roster, when i feel like maxing ones affinity in order to get a FG.