maths help expression

[Bigfoot]

How would i express this as a mathematical equation?

I've got a fastloop looping 150 times.

I only want to effect changes every 3rd loop starting at loop 1, so 1, 4, 7, 10 etc etc.

Onloop my loop
+loop index my loop = ?

Complete brain freeze and i hate maths

[Dynasoft]

loop index("my loop") mod 3 equals 1

index: 0 1 2 3 4 5 6 7 8 9
mod 3: 0 1 2 0 1 2 0 1 2 0

[Bigfoot]

Brilliant, thanks very much.