You are correct - the 3" and 2" is the length. These are the standard diameter bolts for mounting joysticks and such. About the 10-24: The first number is the diameter, and the second is the threads per inch. There is a formula for determining what the actual diameter is, but a 10 is about 3/16 of an inch.
Make sure if you order from DM that you specify you want the nut as well - I ordered a bunch a ways back and only got the bolt...
*Edit*
Shardian is a faster typist...
