Digamos que você tenha algo assim
host1
host2
host3
host..99
Você pode fazer algo assim
awk 'NR%3?ORS=" ":ORS="\n"' hosts
host1,host2,host3
host4,host5,host6
host7,host8,host9
host10,host11,host12
host13,host14,host15
host16,host17,host18
host19,host20,host21
host22,host23,host24
host25,host26,host27
host28,host29,host30
host31,host32,host33
host34,host35,host36
host37,host38,host39
host40,host41,host42
host43,host44,host45
host46,host47,host48
host49,host50,host51
host52,host53,host54
host55,host56,host57
host58,host59,host60
host61,host62,host63
host64,host65,host66
host67,host68,host69
host70,host71,host72
host73,host74,host75
host76,host77,host78
host79,host80,host81
host82,host83,host84
host85,host86,host87
host88,host89,host90
host91,host92,host93
host94,host95,host96
host97,host98,host99