Use is for right now, one person or thing: She is helpful.
Use was for a finished past moment, standing alone: He was helpful yesterday.
Use been only after have, has, or had: She has been helpful since Monday. No helping verb? You want was.
Choosing between them by situation
Before you write, run a quick check with any time word. Take a frame like Last week he ___ helpful. and try one of the three forms. If the time word points to a finished moment, the past form will feel right, and the present form will clash with it.
Writing about right now, one person or thing? Use is: She is at the office. It fits when the situation is still true as you speak, and the subject is he, she, or it.
Writing about a finished past moment? Use was, standing on its own: He was helpful yesterday. No helping verb goes in front of it. Keep the agreement in mind: was covers I, he, and she in the past, while other subjects, like you or they, take were.
Writing something that connects past to present? Use been after have, has, or had: She has been helpful since Monday. Been never stands alone in these constructions; if there's no helping verb, the form you want is was.
The contrast to watch: He was quiet yesterday reports a state that ended. He has been quiet all morning reports a state that began earlier and still holds. Same person, same quality, but was closes the door and has been leaves it open.
So the whole choice comes down to what your sentence says about time. Is points to right now, was closes off a finished moment in the past, and been only ever appears after have, has, or had. That last pairing is where the interesting difference lives: was tells of a past state, while has been says it still continues.