all digits in the number must total to a number divisible by 3 when added up e.g 156=1+5+6=12 which is divisible by 3 you can proove this using other numbers eg 39,24,1107 e.t.c
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It is 3 6 9
3 - 31
There is no divisibility rule for 13 because it is a prime number. If you are thinking: why is there a divsibility rule for 2 and 3 then. Well, i don't know so go look it up on google.
The divisibility rule for 3 is add up the digits and check for divisibility. 7+2+4 = 13.13 is not evenly divisible by 3 so 724 is not divisibleby 3.
633 is divisible by 3 because its digits add up to a multiple of 3.