My guess is were he to run third party, he'd pull about 2-3% of the Trump/Fiorina support and fade quickly.
Carson and his ego have been paid all along to run—first to split the conservative vote and now to stop Trump.
Apparently Carson and his ego have no limits to what they will do, since a third-party run would only bleed votes from the Republican nominee—to assure a continued monopoly of the uniparty.