Enter a sum formula in each of an unknown number of rows - macros

Hello,
I currently have a macro that runs across 31 sheets to copy and paste data from each of them into one place. The number of rows that the macro takes is dependent on the information in the sheets and will most likely be different every time you use it. I am trying to get a summation of each row in the column to the right of the table. This is easily done by typing for instance '=sum(L3:P3) in cell O3 and then dragging the corner down to copy the formula to the bottom of the table, however, there may be hundreds to thousands of rows and, as I said before, it is never the same.
Any ideas on the macro I could use to accomplish this so I don't have to manually drag that formula every time?

Thank you in advance

EDIT: Perhaps adding a For Statement (although I don't know how to use one) that uses the Rows.Count function. But I wouldn't know how to word the 'for-statement' correctly.
Perhaps something like:

EndRow = Sheet31.Cells(Rows.Count, "M").End(xlUp).Row
the counting interger would count the rows (from row 3 to EndRow) and the column would always remain the same (column O)
in each cell it would enter the following :=sum(L3:P3), =sum(L4:P4), ..... all the way to L'Endrow':P'Endrow'

Again, I'm not sure how for statements work, but I think this is the right idea.

Thank you very much, I believe that did the trick.
You have been very helpful.
Reputation Added (not like you needed it)