I have the following array defined as mydata:

. A B
1 1 5
2 2 4
3 3 3
4 4 5
5 5 (blank)
6 6 3
7 7 2


I want to sum column B in groups of 3.
B1:B3, B2:B4, B3:B5 etc... this table can be very large(hundreds of rows long).
I was trying to use Ctrl-Shift-Enter but I must not be doing it correctly with a defined array name.