"AT3" is actually Cyrillic letters "ATZ" which stands for Artiljerijskio Tehnicki Zavod (Artillery Technical Factory), which was in Serbia and made bayonets and other weapons. as near as I can tell that name was used from 1923 to 1931.
We'll need a pic to identify the exact type, but I would suspect that what you have is an M-24 bayonet for a Mauser rifle. Does it look like this?
EDIT : These are commonly called "Yugo M-24 bayonets" and were made for the Yugoslavian M-24 Mauser. Value isn't particularly high on them and they generally sell in the $50 range in good condition if I remember right.
Definitely a neat find. I like old bayonets and I'd definitely hang on to it