Sum Every 3rd Row
Assuming the range is A1:A100
=SUMPRODUCT(--(MOD(ROW(A1:A100),3)=0),A1:A100)
Adjust 0 for row number
0 will sum every 3rd row
1 will sum every 3rd row starting with row 1
2 will sum every 3rd row starting with row 2
Adjust 3 for number of rows to be summed