Formula calculation in BEx

I have a report:

Item//Per1//Per2//Per3

income//25//36//52

costs//21//23//09

adj costs//4//13//8

Now I need a row like the below:

LTM adj costs//25//19//11 (25=4138 i.e. adding all the adj costs from present to last 2 periods) similarly 19 and 11 assuming the previous period adj costs values are 2 and 5.

NOTE: Items are from the structures in the rows and periods are offsets for Posting periods in the column.

How can I calculate or define the LTM adj costs.

Please let me know if I am clear.

Thanks,

Nikki