Did you test?

Row(A1) = 1

And since it's relative reference. As you move down you increment by 1.

So, if you enter following formula in C3 and copy down... you get the result you asked for.
=IF(ROW(A1)<=A$1,1,IF(ROW(A1)-1<A$1,A$1-ROW(A1)+1,""))

Or using 3rd row - 1st row = 2
=IF((ROW()-2)<=A$1,1,IF((ROW()-3)<A$1,A$1-ROW()+3,""))