Cash flow based on start, length & frequency...help!

I can't really see your model layout... I assume it's annual?

If you have a date input I assume there must be a row with dates for each column. ...or is it just matching a year?

In my models, I create a row for the contract "period" and then based on the start date I add logic so that row contains zeros before and after the start date, and counts 1 to n for the specified contract period.

Once you have your "period" row functioning correctly, you're on the right track with the MOD function. Set it up so it returns zeros and ones. The ones being every third year =--(MOD(n,3)=0) where n = period

Then just multiply the resulting binary switches by your recurring value.