Record a 3 hour live webcast video & audio

QuickTime X (part of Snow Leopard) does include a screen capture feature. I wouldn't use it on such a long file.
http://www.ambrosiasw.com/utilities/snapzprox/
What I use to make my recordings. Check your HD free space before beginning.

It would depend on the resolution (dimensions) of the file, the frame rate, the codecs used during capture and the amount of compression.
There is no "rule of thumb" for screen recording because the options are so variable and, since your source is "live", you don't have the option for reducing the file size until the capture is complete.
Sounds like you nee more storage space before you begin.

How much hard drive space do you have???

Depending on the dimensions (xy), acquisition codec and final codec for the video your looking at some major hard drive space.

I believe Snapz Pro X uses acquisition codec and then transcodes that video to a final codec/format. IShowU is another screen capture program. It transcodes on the fly, major CPU usage. (I have both of these programs.)

This is something to should have been setup and tested a few days ago. All you need is a small hiccup during recoding and you'll end up with nothing but garbage at the end of the recording. If you recoded to your system hard drive and fill it up during recoding you could run into some problems with you system hard drive. I wouldn't advise recording to your system hard. Recoding to a USB external hard drive won't cut the mustard, to slow. You'll need something like a external 800 firewire hard drive.

Three hours of recoding won't be a easy task.

Perhaps for the next webcast, here's a quick tutorial how to set up [VLC|http://www.videolan.org/vlc> for recording a Video stream to a file.

http://www.wyodor.net/VLCStreaming/

I've never used the applications available to you so my answers could be wrong.
SnapZ can record (audio and video) of any source that plays back on your computer (except "protected" media sources).
It lets you set the "dimensions" of the capture area. A "window" or an "area" on the sceen. You can change the size of the capture (lower file size) by changing the Display settings on your machine.
At 800X600 Display resolution the capture area would be very large resulting in a large file size. At 1920X1200 (my iMac settings) the capture area would be much smaller.

It can "record" (audio and video) what displays (and plays through) your machine.
Basic answer:
If you can see "it" and hear "it" SnapZ can "record" it.
Check the Audio MIDI Setup app (Utilities folder) for more answers.

Yes, what about VLC?

http://discussions.apple.com/message.jspa?messageID=10424573&tstart=0

To record you need the URL of the live stream.

It's the same URL you need to view video streams in QuickTime.

The difference is that you can save the videostream with VLC.

That's what I explain in my tutorial.

Have you already tried something?

Perian is a codec pack that allows playback of various videos that QT does not natively support ( AVI files and Flash mainly) and doesn't record anything.

If you want to record a 3 hour conference and do a good job get Snapz Pro. You can try QTX and stop the recording every 30 minutes (makes a new file when you restart) you can try that but I'd practice first. The VLC option is another idea but again, try it first.