Average Calculated Field on Pivot Table

[adrienne0914]

I am trying to insert a calculated into field my pivot table so that it gives me the average for the last 3 months in the last column. When I use the formula =AVERAGE(Last 3 months), it inserts a formula between each column, rather than giving me an average to the right. I'm sure there is a simple answer, but I struggle with adding formulas to pivot tables.

[TABLE="class: grid, width: 500"]
<tbody>[TR]
[TD]Sum of Animals[/TD]
[TD]Column Labels[/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD][/TD]
[TD]Last 3 months[/TD]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD]Row labels[/TD]
[TD="align: center"]Feb[/TD]
[TD="align: center"] Mar [/TD]
[TD] Apr [/TD]
[TD] 3 Mo Avg [/TD]
[/TR]
[TR]
[TD]House Pets[/TD]
[TD="align: right"]15[/TD]
[TD="align: right"]123[/TD]
[TD="align: right"]72[/TD]
[TD="align: right"][/TD]
[/TR]
[TR]
[TD] Cats[/TD]
[TD="align: right"]4[/TD]
[TD="align: right"]5[/TD]
[TD="align: right"]7[/TD]
[TD="align: right"][/TD]
[/TR]
[TR]
[TD] Dogs[/TD]
[TD="align: right"]5[/TD]
[TD="align: right"]100[/TD]
[TD="align: right"]20[/TD]
[TD="align: right"][/TD]
[/TR]
[TR]
[TD] Parrots[/TD]
[TD="align: right"]6[/TD]
[TD="align: right"]18[/TD]
[TD="align: right"]45[/TD]
[TD][/TD]
[/TR]
</tbody>[/TABLE]