Tente isto:
$ cat file
1 2 3 4
5 6 7 8
1 2 3 x
a b c d
5 6 7 y
Olhando para os 3 primeiros campos como "chave", queremos descartar as linhas 2 "1 2 3" e 2 "5 6 7"
awk '
{
line = $0 # remember the original state of this line
NF-- # forget about the last field
}
!seen[$0]++ {print line} # if the "new" line is unique, print the "old" line
' file
1 2 3 4
5 6 7 8
a b c d