According to sources, Trump has agreed to sign the GOP loyalty pledge that will bind him to endorse the GOP, and would preclude a third-party run. Source:
The rationale is that they have treated him fairly. He’s willing to sign it to avert any problems with is state filings – South Carolina asked for a pledge to support the nominee if you’re on the ballot. He’s been treated fairly, like any candidate. … And it takes away a line of attack for debates.