Are the Cam Jansen mysteries too easy for a third grader? I'm afraid I can't recall how old DS was when he was plowing through them.
Encyclopedia Brown was another favourite, although I think the stories are all fairly short.
The Screech Owls series by Roy McGregor is about a kids' hockey team that encounters mysteries and adventures on their travels to tournaments.
The Roman Mystery series by Caroline Lawrence is another possibility. They are set during ancient Rome. The first in the series is The Thieves of Ostia.
Hoot by Carl Hiassen has a mysterious aspect to it, as well as a good environmental message. Other titles are Flush and Scat. They are longer novels and probably targeted at slightly older kids, in the 10 to 13 y.o. range.
Holes by Louis Sachar has a mystery aspect, although it isn't a typical detective novel, more like an adventure tale. Again, it's for a slightly older reader.
Thought of a few more, although they may be a little challenging depending on his reading level -
The London Eye Mystery by Siobhan Dowd
The Mixed Up Files of Mrs. Basil E. Frankweiler by E.L. Konigsburg
The Westing Game by Ellen Raskin
Thanks for the suggestions. He has read one of the Three Investigators books and liked it. I've requested some more through our library system. He reads Encyclopedia Brown books when he's waiting for Hardy Boys books to come in. He can read several of those in a day, but likes them. He tried Hoot a while back and it didn't suck him in like mysteries tend to do, so he gave up on it. May try it again. I'll look into some of those other authors, too. He informed me last night that he isn't in the mood for Hardy Boys books right now and wants to take a break from them.