copying and pasting average formula

HAving difficulty with averaging data.

Working with large daata block and want to average data in 5 minute intervals to form 15 intervals to match up with over data.

ie
1 average(A1,A2,A3)
2 average(A4,A5,A6)
3 average(A7,A8,A9)
4
5
6
7
8
9

want to be able to simply scroll it down the column but when trying the formula goes like the following

1 average(A1,A2,A3)
2 average(A2,A3,A4)
3 average(3,A4,A5)
4
5
6
7
8
9

Is there an easy fix to this?

Cheers for any help