Formula to add cells based on criteria

Hello:

Please refer to attached file.
I have monthly data as shown in Sheet1.
Pls refer to sheet2.
Depening on value in cell c1 (month #), i need formula in in cell D3 and D4 to add 3 months of data for items listed in column A.

I have shown manually entered result.

Let me know if you have any questions.
Thanks.
Riz