That's a very good question...
Cmok is like the English "kiss(es)". It doesn't literally mean a kiss but more like making the kissing sound (if you know what I mean)
Poljubim te sounds like a part of a sentence so, unlike the other two, can't be used separately. It means something like "I kiss you". E.g. "poljubim te i ti se rastopis" (I kiss you and you melt )
(po)ljubiti = to kiss
Ljubim te is used to end a conversation and although I suppose some people might use it casually, I'd say, even though its literal translation is "I'm kissing you", it has a bigger meaning than that, i.e. it means "I love you" (often used in poetry and probably derived from "ljubav"= love). Most likely, it's something like "Love ya" (easier to say than "Volim te"=I love you).
Ljubim te doesn't have to mean romantic love, you can also say that to your mother, sister...