It is a slang and abbreviation of 열심히 공부중이야 Mostly elementary age kids first started to put letters like 셈,삼 at the end of the sentence on the Internet or in an online game before. People, nowadays, quite don't use it, though you will sometimes find out people, regardless of their age, using it among friends.

It means "I'm studying hard". 열공 is a shortened form of "'열'심히 '공'부하다". And 중이다 means "doing st now". Using "삼" instead of "다" at the end is a slang among teenagers. It's an informal way. ex) 좋다 -> 좋삼 즐겁다 -> 즐겁삼 알았다 -> 알았삼

Are you studying hard?