Return every nth value in column

Hi all,

Basically I have a HUGE dataset, which i am trying to minimise by extracting every nth row, (this can vary - but for now assume every 5th row).

I have attached an example of the spreadsheet...

The columns I have are:
A - x value
B - y value
C - height

I want to duplicate rows A to C, for every 5th row, into (for example) columns G-H. The values in each row are linked (i.e. h corresponds to the coordinate (x,y)) - so these can't become mixed

Cells A3:C3 to G3:I3
Cells A8:C8 to G4:I4
Cells A13:C13 to G5:I5
...and so on.

I have tried to use the OFFSET function, =OFFSET($A$3,ROW()*5,0), but it doesn't return every 5th row, and i'm now too sure what it is doing!

I have >1,000,000 rows of data in case this affects the formulae

I hope somebody can help!

Thanks & Kind Regards,

Rachel