So... technically speaking... there is a way that may be legally dubious, but it's not expressly, objectively against the constitution and hasn't been ruled on in any court. The term limits in the constitution do expressly forbid being elected to more than two terms or more than once if you have served more than half a term that your weren't elected president (like if you took over as VP after the President dies/resigns like LBJ and Ford). But there is nothing that prevents him from taking a position in the line of succession. He could even run as VP for another candidate. If he did so, even if they said publically that it was their intention for the presidential candidate to resign immediately once taking office and for Trump to become president again, there doesn't seem to be anything preventing that. It certainly defies the spirit of the law, but not the letter of it.

Alternatively, suppose the courts did rule that approach illegal. They just move the goal post. He could take the Speakership and be next in line after the VP. He doesn't even have to win an election to do that. Speaker of the House doesn't have to be a House Representative. The GOP could nominate him to the house and confirm him without so much as a public poll. Then, should the Republicans win the Presidency, they could simply resign both President and VP and then Trump assumes office again.

None of that expressly breaks the law. Which is fucking nuts. But they left that loophole open by writing in the language of the amendment not that a person "cannot serve more than two terms as president" but that they cannot be "elected" to more than two. Given that there are many routes the presidency that do not require you be elected to that role, that seems like a glaring omission that only requires a conniving group of conspirers willing to evacuate positions of power for Trump. Will that happen? I don't know. But can it? It does seem so.