Amendment XXII says someone can't be elected President more than twice - but it doesn't say anything getting in the backdoor, so to speak.

I suppose in theory the Supreme Court, being a law unto itself, could arbitrarily decide that a Vice -President automatically succeeding a President who had died or resigned was to be deemed as "elected". I'd think that would be a serious contortion of the normal sense of the word, but that wouldn't necessarily stop them.