Bit of an odd question

I'll soon have 4 5* (have 24 medals on current event)

10 4*

So squad wise I can nearly make 3 squads already with 4*+

My question is...are any 3* worth keeping and evolving? They easily get 4 slots and level 5 skills....so I was wondering are there any real gems amongst the 3* that are really worth keeping or do they phase out gradually over time?