Your best bet is to use the new IMAQ Vision 7, which was released last Tuesday, I believe. It now has native support for writing compressed AVI movie files. These VIs are used the same way the Write BMP.vi and other file I/O VIs are used in IMAQ Vision 6.1.
Writing 3 files simulateously should be fairly straightforward. If you can get it to work with one camera (img0), then just copy and past the code two more times for img1 and img2. Also write to three different AVI files. That's all you should have to change! Bandwidth to the harddisk may be another matter all together...
Kyle V