The first two are similar enough. All tense would be completely tense. So tense is more tense but not completely tense. So there is a difference: Completely = all (do not get completely tense) = physically relax some So = to this level of (do not get to this level of tenseness) = physically relax some The third one is not the same. You could tense your body without nervousness. You could be nervous (in theory) without tensing your body or muscles. Do not get nervous = mentally relax

"Don't get all tense." = SAMPLE SITUATION: I'm going to give you some bad news. Promise me you won't get all tense when you hear it. (tense = get angry or stressed out) "Don't get so tense." = SAMPLE SITUATION: Okay, seeing a rattlesnake might get anyone a little nervous. But don't get so tense (meaning: you are letting your muscles lock up or become immobile). The poor snake is as scared of us as we are of it. Just keep your distance. "Don't get so nervous." = Some tightening of the muscles is expected when you get nervous. But if you're really nervous or scared, I expect you to be trembling. But if you're really tense, I expect you to be stiff.

Yes, tense is more like physical nervousness or fear, but the sentences have practically the same meaning.

The meanings are identical for all practical purposes.