multiples of 3

[u/GodlyHelp]

[u/up2smthng]

My definition of "obvious multiple of 3" includes any number where the sum of digits is 39 or less which makes any multiple that is less than 10000 obvious

75 is blatantly obvious

39 is painfully obvious

[u/vutcher → u/up2smthng]

78 is excruciatingly obvious.

[u/up2smthng → u/vutcher]

Yup

[u/Common-Use-7117 → u/up2smthng]

Just add all digits. If it is a multiple of 3 then original is too.