tidal-locking seems good to me. Easy to recognize.
spin-orbit-resonance might be more technically correct, but I doubt people will recognize it easily enough for the tag to be useful. Also, for most questions that really needs one side to the sun, resonance tag is a bad choice. Other resonances (if exists) can't give results people expect from locked planet. So if other resonances exists, it is not a synonym, but rather a broader tag. I don't recall any single question that would need this broader definition, and all existing ones call for tidally locked and nothing else - anything else would be unsuitable for OPs' purpose.
Note that challenging question's premise is already considered acceptable, so this in no way prevents anyone from answering like “you’ll really get 3:2 in that case”.