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 maxed 3* where I have a champed dupe?

I posted this in another thread, but think it makes more sense here.