Fixing function results
 

I have used an OFFSET function to produce a result which will always count the last 3 values in a column. This column increases in length on a monthly basis, therefore I am always totalling the last 3 months values.

I would like to check the total each month and, if it is over a certain value I want this recorded in another cell eg the word Yes could appear if the 3 monthly total exceeds the stated value. I can do this easily with an IF function. However, I want this result to be fixed, so that when the next month's figure is added (which may not give a total excedding the stated value) the cell content will not reflect the change.