Let's say Sheet 2 has many columns of figures for many months (column A is
January, column B is February and so on).

I want a formula in Sheet 1 that sums up three months at a time. For
example, Sheet1.A1 = SUM(Sheet2:A1:C1), Sheet1.B1 = SUM(Sheet2:D1:F1).

I would just type these formulas in, but I could potentially have to do this
for 24 quarters across (copying and pasting one forumla across doesn't work).

Can anyone help me with this?

Thank you,
Judy