An array formula to sum every 3rd.
Enter it with Ctrl-Shift-Enter.
For ease of use:
The first amount to be summed should be in a row that is divisible by the Nth factor.
Example Row 9/3
One can insert or delete row to accommodate this.
=SUM(IF(MOD(ROW(A1:A17),3)=0,A1:A17))
or more generic
Nth 3 or alternative
rD range of numbers
=IF(Nth=0,0,SUM(IF(MOD(ROW(rD),Nth)=0,rD,0)))