welcome to the forum.
It's helpful if you could give specific examples of what you are asking. Like what cells have the values, and so on. However, here's one idea:
=choose(countif(a1:a3,">=90%")+1,0,0,75%,100%)
if the cells you are looking at are a1, a2 and a3, then the above formula might work for you. Let me know.