You need some kind of switcher. I think this route will more get expensive then what your looking for. Production video switchers don't come cheap. I would look at JVC or Sony and see if they have something for under $2000. After you get a video switcher, get an audio board and mics and then pipe the audio and video into your mac with a Analog - Firewire converter. You could then set iMovie to record and let it caputure the output of your switcher.
Here's some ebay ones that can get you started as what to look for. Remember what your talking about is what people pay studio time for. Your talking about building a studio, not so hard for some but get's expensive. I would personally vote for 3 cameras and then do the switching in post, either iMovie or Premiere.
The best way to go but you need other hardware to get these up and running, ie reference signals, time base correctors, etc.
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=1394551772
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=1394852694
This would be your best bet if you don't have a big budget. You wont get disolves, etc. but you can do all that in iMovie. All you would have to do is hook the cameras up, then put the output into your firewire converter.
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=1394620993
Watch out for some of the other routers like one listed above. Some of the $100 ones you'll see on ebay won't switch video on the "vertical interval" (where the top of the frame starts) meaning the video will jump everytime you switch sources. I know the one here does switch on vertical interval.
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=1394620993
Hope this helps,
Brad