Well, without knowing much about video frequencies, I was talking in the original reply about a plain ole signal splitter, I.E. your video out from the graphics card would go to this splitter, which duplicates the signal to three other outputs.
It's all coax.
The same setup we used in college in the dorms to yank free cable TV from the guy down the hall
And yeah, you're right, I don't guess each terminal would have a mouse with one graphics card. After a little thought I think the automatic display through the video out would be the easiest solution. Might not be very "nice looking" to the end users of these three displays, but it'd work, and would be pretty fool proof. Instead of trying to admin a mail server for 3 or 4 users, you just have to set up the client to spit messages at video terminals.
Xayd