Subjunctive after certain verbs

Solution 1:

This is an area where usage has been changing, and may still be changing.

Many people use the subjunctive form after verbs that demand or suggest: demand, insist, recommend, advise, require; but others don't. That form is more common in formal than informal contexts, and more used in American English than British.

(Besides the simple present, which some people use, and others object to, there is also a form with would or should, which I think everybody regards as grammatical, but the subjunctive users prefer their subjunctives).

The subjunctive with verbs of thinking (suppose, assume, guess) is now pretty well obsolete. I don't think any native speaker would use your second or third examples, unless perhaps they were being deliberately archaic.