counting every 3 rd row

[ezil]

The formula "=countif(a1:a1000,"A??")" is working, but i have to count every
3rd row from a1 to a1000 like a1,a4,a7 etc., How to write formula to do this
task.
Thanks for reply

 

[joel]

=SUMPRODUCT(--(MOD(ROW(A1:A1000),3)=1),A1:A1000)

 

[ezil]

=SUMPRODUCT(--(MOD(ROW(A1:A1000),3)=1),A1:A1000)

Thanks for the formula but where to put the condition How the formula like
with the condition

 

[Patrick Molloy]

i had to do it like this
in column B put
=MOD(Row(),3)

this gives a 0 for rows, 3,6,9 ....
then in another cell

=SUM((B1:B100=0)*(A1:A100="A"))

entered as an array formula

this, B1:B100=0 , is what gives values for every third row and this
A1:A100="A", counts A's

the array formula thus counts where col B=0 and Col A is "A"

 

[joel]

=SUMPRODUCT(--(MOD(ROW(A1:A1000),3)=1),--(A1:A1000="A??"))