The big problem with this is that each USB mic is digitised on its own with no reference to any other source of sync - that's what USB normally does. And the problem with this is that you can only have one source of digital sync into any DAW. What you'd need would be a means of taking several out-of-sync sources, and synchronising them within your PC. Now, amazingly enough, there's provision within the latest version of the USB spec for this to happen - you could, using the right mode, get all of your USB mics synced - Atmel can do this in one of their AVR processors. You need this absolutely, otherwise you'd get loud clicks and pops even when they're running at the same rate if the clocks aren't actually physically locked.
Slight snag though. There's not a manufacturer around who's done it, to the best of my knowledge - there's really very little call for it at all, and it would cost them a significant amount more to implement on what's seen as a cheap interconnection solution. So I'm afraid you're out of luck with this one.
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.
Bob Howes wrote:
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.
Possibly you might find that if you use two identical USB2 mics where the sync is derived from the connection clock, the much tighter clock tolerance (the rate is 480Mb/sec +/- 500ppm) would result in the sample clocks for each encoder stayed within a sample of each other, even though they weren't technically locked. I wouldn't rely on using a system like this for anything, personally...