3 only
7 only
1 only
1, 4, and 7
do you mean which digit is added to 371 so that it is divisible by 3
test for 3, sum of digits divisible by 3
3 + 7 + 1 = 11
if you add 3, 11 + 3 = 14 not divisible by 3
if you add 7, 11+ 7 = 18 divisible by 3
if you add 1, 11+ 1 = 12 divisible by 3
if you add 1, 4, 7 divisible by 3
