From many places where we can pick up Korean, I’ve always seen 이/그/저 used as 이것/그것/저것. When I heard that there were short forms, I was pretty pumped because it might make things easier when I translate twitter messages. Since colloquial is usually used.
이것이 (this thing + subject marker) = 이게
그것이 (that thing + subject marker) = 그게
저것이 (that thing over there + subject marker) = 저게
Hope this will be useful for learners!