On Sat, 03 Mar 2012 21:19:48 -0500, Ron Rosenfeld wrote:

One way, by setting a range greater than your expected number of entries.
FirstRow is the first row number that you want to add. e.g. 1 for A1, A4, A7, etc
2 for A2, A5, A8, etc.

=SUMPRODUCT((MOD(ROW($A$1:$A$1000)-FirstRow,3)=0)*$A$1:$A$1000)

Actually, the above formula will only work if all of the entries are numeric. If some of the entries are text, then the above formula will give a VALUE error. To avoid that, you can use the following formula:


This formula must be **array-entered**:

=SUM(IF(MOD(ROW($A$1:$A$1000)-FirstRow,3)=0, $A$1:$A$1000))
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To **array-enter** a formula, after entering
the formula into the cell or formula bar, hold down
<ctrl<shift while hitting <enter. If you did this
correctly, Excel will place braces {...} around the formula.