awk number rows from 1 to 3 in file

Shell Programming and Scripting
1

I have a file with the following format, i need to change de field9 each 3 rows to renumber field9 with gpo1, gpo2, gpo3.
I need to use awk

Original file

field1    field2    field3    field4    field5    field6    field7    field8     gpo3
field1    field2    field3    field4    field5    field6    field7     field8     gpo76
field1    field2    field3    field4    field5    field6    field7     field8     gpo4
field1    field2    field3    field4    field5    field6    field7     field8     gpo5
field1    field2    field3    field4    field5    field6    field7     field8     gpo88
field1    field2    field3    field4    field5    field6    field7     field8     gpo99

New file

field1    field2    field3    field4    field5    field6    field7     field8     gpo1
field1    field2    field3    field4    field5    field6    field7     field8     gpo2
field1    field2    field3    field4    field5    field6    field7     field8     gpo3
field1    field2    field3    field4    field5    field6    field7     field8     gpo1
field1    field2    field3    field4    field5    field6    field7     field8     gpo2
field1    field2    field3    field4    field5    field6    field7     field8     gpo3

thanks in advance

2 
awk 'NR<=3?s="gpo"NR:s="gpo"NR-3{$NF=s}1' OFS=\\t file
3 
awk '$NF="gpo"i++%3+1' OFS="\t" infile
1 Like
4

thanks Scrutnizer works perfectly