I want to calculate rolling average of the previous 3 months.

I was going to do this using windowed lag functions

(

(LAG(SUM(`Sales Amount`),1) OVER (ORDER BY `YEARMONTH`)) +

(LAG(SUM(`Sales Amount`),2) OVER (ORDER BY `YEARMONTH`)) +

(LAG(SUM(`Sales Amount`),3) OVER (ORDER BY `YEARMONTH`))

)/ 3

Each Lag works individually but any time I try to do any computation between them the card breaks and throws the error:

An issue has occurred during processing. We are unable to complete the request at this time. 

Any idea of how I can get around this? Or is this no longer possible? I feel like I have done this in the past.