In Tableau 8.1, we can set the type of aggregation for the Total rows, which makes life much simpler.
What you want is possible in earlier versions, but it takes some work (assuming I understand correctly--you want to see the average, not the sum, of the columns in the bottom row--right?)
Some suggested reading, which discusses several ways to approach this problem:
I just only want to show de average.
Month 1 3287
Month 1 889
Month 1 3146
I need to show the average at the bottom of the column.
Is it too difficult????
Nope, it is very possible--the link I provided will show you a variety of ways to do this in Version 8.0. Again, if you can upgrade to Version 8.1, there's a menu option that will allow you to change the aggregation of the total rows.
Jonathan's blog also demonstrates this in 8.1: Tableau 8.1 Two Pass Totals | Drawing with Numbers