I've been watching some professional play recently, and I tend to see tanks like J4, Vi, Olaf, even Gnar, etc. getting these early warmogs instead of items like Lock, Randuin's, Sunfire, etc. Why is that? When and when is it not a good idea to get Warmog's as second or third items?
It helps laning phase a lot so it's a good early item. Randuin's is unfortunately very gold inefficient, and so is the frozen heart but at least it has great benefits and it's components are very useful. Get this item if you think your opponent can push you out of lane fairly easily. With this, all you have to do is play safe and you'll regen to full health.
idk.. if you noticed them getting warmogs you should also notice who they were up against
these wannabe 
s
jealous of
want to be
MUNDO
Warmog gives 800 hp. Turns out it's 100% of your health early game and 50% later. The warmog is always gold efficient and slot efficient no matter what but it make you scale more if you can squeeze it early in your build. I believe that why you see lcs players doing it.
When and when is it not a good idea to get Warmog's as second or third items?
When you can 
te it.
Warmogs second on vi is bought to increase the size of her passive shield making her mu tankier early
