Answer: 6 and 12 are the only multiples of 3
To check if it is a multiple of 3:
add the digits and then divide by 3
if there is no remainder it is a multiple of 3
1+2 = 3
3/3 =1 no remainder it is a multiple of 3
20 = 2+0 = 2
2/3 = 0 remainder 2 not a multiple of 3