Happens how, exactly?
How does President Trump abolish term limits? He can't do it through an executive order. He can't do it by signing a bill into law (because Congress can't actually pass such a bill).
The best he could possibly do, to get elected to a third term, would be to campaign for a third term in spite of his ineligibility to serve a third term. He might get as far as having his name on ballots in some states, even though he's not eligible. Between that and write-ins... What? Suddenly he's abolished term limits?
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Or he'd just refuse to step down, but that wouldn't make him a third-term president. That would just make him a non-president trespassing in the White House until the Secret Service removes him an hour later.