1. Usually used to express "love", or just "luv". It's usually less important than an actual "I love you", more like something you say at the end of an IM conversation to someone you
don't necessarily "love". PS: the more 3's you
add, the more you "love" the person.
2. I find that it also looks like a mouth to the side, like a
kiss, the "3" being the lips. PS: the more 3's you
add, the more kisses you'
re giving.
3. In other uses, one can also say that it means "less than three" or "smaller than three" but that is more mathematical.