I'm trying to write a function that will copy from one row to the next one
down, but move the target range a greater distance. Here's the scenario.

A1=10
A2=20
A3=30
A4=40
A5=50
A6=60
A7=70
A8=80
A9=90
A10=100
A11=110
A12=120

B1=SUM(A1:A3) = 60
B2=SUM(A4:A6) = 150
B3=SUM(A7:A9) = 240
B4=SUM(A10:A12) = 330

Do you see how the target ranges in column B skip? I'm only going down one
row, but I want the data collected to move down by three rows. My question
is this: Is there a way to write the function in cell B1 so that I can copy
it and paste it to cell B2 and on down without having to re-write the range
of cells in the parentheses?

Thank you!