kog'maw DOES NOT BUILD TRIFORCE ANYMORE
ever since tri lost zeal component there is little reason to build it on kog'maw. zeal/phage/sheen force was good because it gave kog'maw stats to make it to midgame. the MS from phage with the tankiness, the zeal crit + MS, sheen proc and mana.
zeal in triforce was especially good because kog'maw could build it to later pair it up with PD and IE. but now it gets you to midgame but doesn't help you scale besides reaching max AS and 20%cdr.
you could argue that cdr is useful on kog'maw to lower all cooldowns but Q is used selectively and E just costs too much mana. W used to be a "free" sheen proc.
R was good with sheen for 2 reasons: 1) r is mana dependent and with sheen it'd give you access to almost 3 consecutive R casts AND 2) has roughly the same cd+cast time with sheen proc time. R used to do more flat damage which was good for poke but now its utterly useless vs champions above 15% and even below 15% its a terrible execute (unless you're ap kog).
now, i'm not saying kog'maw cannot build triforce and do well but what i am saying that there is a waste of an item slot and stats for an item that doesn't get him past midgame. OG tri did that tho RIP