J @ Y wrote:
The offset equation returns an range(ie: $A$1:$A$3), but if I put a 1+ in
front of the Offset function (=STDEV(1+ OFFSET($A$1:$A$3,
3*(ROW(B1)-ROW($B$1)), 0) ), it gives me a #Value error.

I suspect you forgot that this is an array formula. Select the cell,
edit it or press F2, then type ctl-shift-Enter.

That should eliminate the #VALUE! error. But that does not mean the
formula will return what you intended.
Computer geeks like me call this GIGO ("garbage in, garbage out").

Why are you adding "1+" to the OFFSET() result? That is, what do A1:A3
contain (i.e. what is the form of the "monthly data"); and what do you
expect by adding 1 to them?

I am not saying necessarily that you are doing anything wrong. But
since your comments reflect a misunderstanding of what OFFSET() does
exactly, I wonder if your "1+" is simply building upon that
misunderstanding.