Copy cell from every nth row

In B2 please try:

=offset(A$1,3*(row()-1)-2,)  

As the formula is copied down the offset row relative to A1 increases by three, with no column offset. In the second (starting row) 3*(2-1)-2 is 1 so the A2 value is returned. In the second output row 3*(3-1)-2 returns 4, so the A5 value is returned, and so on.

Alternatively, also try:

=filter(A2:A, mod(row(A2:A)+1,3)=0)