May I expand on Steve's reply and respectfully suggest that you DON'T look at USB mics. They're designed to be a quick plug and play solution for simple recording needs like podcasts but quickly get outside their comfort zone as soon as your needs go up in terms of either the number of mics or the quality of the recording.
If you don't need the facilities of a full mixer, you'll find the USB audio interfaces are a relatively inexpensive way to get ANY microphone into your computer.
You said "2 or 3" mics but this small difference can be fairly big in terms of cost. Basic two input interfaces are extremely common; as soon as you jump to four inputs (I don't know of any 3 input devices) the cost jumps substantially. If you CAN live with two mics, there are tons of solutions--I recently bought an M Audio M Track device for a specific project and it seems to have decent audio performance, all the facilities I need in terms of monitoring and nice stable ASIO drivers--and it was cheap. However, there are tons of others to look at.
As for you original question, I've heard of a few people who have managed to get a couple of USB mics working at once and a whole lot who haven't managed it and wished they hadn't invested in USB mics. It's worth noting that this isn't an Audition function--it just takes what it's given by Windows or your interface--but, rather, depends on how audio is handled in your machine and how the mics you choose use their drivers. It's more trouble and risk than it's worth.