If you've reached this point, when you almost have an entire team of at least 4* knights, then 3* are a drag and just cost you money. You should use them to fill the missing spots, but soon you'll have more than enough units for that and probably start replacing some of your 4* units.

You should keep them however, you never know where they may come in use for, just look at this event we had to use our 2* units.