PvP Essentials Tracker? Selling maxed 3* advice.

Is there a thread/site that tracks the history of what characters have been required for each PvP  event?  I've reached a point where I should start selling my maxed 3* to get get Iso for champing 4/5*.   I figured I'd start by selling ones that have been used in PvP recently, hoping they won't come up again too soon.  Anyone have advice on best way to handle selling 3*?