I don't keep 3* players in my team.
4* if they are old, over 24-25 y.o. (so they don't gaining much) also should be sold because they cannot help much in the next season.
(but players over 27 y.o. 5*, can help much for a couple of seasons).
Better to sell them before semi & finals because some managers may need them for their games.
Better days for selling is Friday evening to Sunday evening.
Of course many things depending from the wealth of the team so everyone has a personal style.