Josh Marshall thinks there's a good chance that Trump will try to get out of debating Hillary Clinton and that one of the excuses he'll use is to refuse if the debates don't include Gary Johnson and Jill Stein, both of whom will probably be excluded by existing rules. My guess is that Trump will begin insisting on that and all sorts of conditions, ostensibly out of fairness: He won't accept this moderator or that venue or those time restrictions or the composition of the audience, etc. The idea here is to show he's in command…and that Hillary is going to have to show up and participate in a format dictated by him, rather than the two of them entering an arena neither controls.
That seems to be a dominant trait in Trump: He never plays your game. You have to play his. In interviews, he answers the questions he wants to answer, not the ones you want to ask him. I'll bet when he engages in business negotiations, he won't go to your office. You have to come to his and sit on his furniture, surrounded by his photos and his loyal staffers and you eat and drink what he wants to serve you.
If he gets every rule change and concession he wants for the debates, he'll participate because he'll figure he's already won by getting those concessions. If he doesn't "win" that scuffle, whether he'll debate or not will depend a lot on the polls and public pressure. He can't afford to have "he's afraid of Hillary" become too popular a chant.