Quarterly Average

I need help with a calculation in my form. I have a form named families. IN this form I have 12 check box's, one for each month. I would like to set up another box which would take the average of the past 3 months and tell me what percentage of the time the box is checked. For example, since it is december, I would like a box named quarterly average to look at the past 3 months, obviously september, october and november, and tell me in percentages what the percentage is that the past 3 months check boxes have been checked. This is the basic code which I created for my unbounded box, but I want it to be dynamic, so that it recognizes what the month is today and tells me automatically what the percentage is.

Control sourceis set to =Abs(([Sep]+[Oct]+[Nov])/(3))

Thanks,

Tim