awk NF

echo AIX.PI6002.090316.tar.Z | awk -F "." '{$NF="";$(NF-1)=""}{print $0}'

echo "xxx.xxxx.xxxx.tar.bz" | awk -F. '{print $1"."$2"."$3}'

echo AIX.PI6002.090316.tar.Z | awk -F . 'NF-=2' 

echo AIX.PI6002.090316.tar.Z  | awk 'NF-=2' FS=. OFS=.

echo "xxx.xxxx.xxxx.tar.bz" | awk -F . 'NF-=2' | sed 's/ /./g'

echo "xxx.xxxx.xxxx.tar.bz" | awk -F"." '{print $1"."$2"."$3}'

echo AIX.PI6002.090316.tar.Z | awk -F "." '{for (i=1;i<NF-2;i++) printf $i".";print $(NF-2)}'

echo AIX.PI6002.090316.tar.Z | awk '{print substr($0,1,match($0,/\.[^\.]+\.[^\.]+$/)-1)}'

echo AIX.PI6002.090316.tar.Z  | awk 'NF=NF-2' FS=.