Got 21

Funny fact, the number 100, in any base (>2 obviously) is not prime (updated)

In base 10, the number can only be prime if it ends in 1, 3, 7, 9 (where 10-1 = 9 and 10 - 3 = 7) - condition necessary but not sufficient of course

If all digits sum to a multiple of 3 it is divisible by 3, of course (because 10^X mod 3 is 1 for any integer X)

For 7 you want 3x the 2nd digit (21 - 2x3 + 1 = 7, divisible by 7) and 2x the 3rd digit (so 119 - 2x1 + 3x1 + 9 = 14)