Two numbers $90$ and $m$ share exactly three positive divisors. What is the greatest of these three common divisors?

90 =1 | 2 | 3 | 5 | 6 | 9 | 10 | 15 | 18 | 30 | 45 | 90 (12 divisors)

9 = 1, 3, 9 (3 divisors)

9 would be the greatest