Answer:
4 and 9
Explanation:
A number will have exactly three (3) divisors if it is the square of a prime number.
Domain of interest
We can find the numbers of interest by looking for primes whose squares are less than 20. That is, we're interested in primes less than √20 ≈ 4.4.
There are two (2) such primes: 2 and 3.
Numbers of interest
The numbers we want are ...
2² = 4, has divisors 1, 2, 4
3² = 9, has divisors 1, 3, 9
The numbers 4 and 9 have exactly 3 divisors.