Sometimes, certain 3-key combinations won't work.
The reason is because despite each key being individually laid out on the keyboard, the circuitry that detects key presses actually uses one wire to detect key presses for more than one key. So imagine that when you press a key, a signal goes down the wire saying "hey, the user pressed a key" (note that the message doesn't say what key is pressed, only that
a key is pressed). Now if you have multiple keys on the same wire and you press 2 or more of them down, the keyboard won't know that you pressed the others down. Basically your 2nd key press is lost because the wire is already in use telling the computer that the 1st key is held down.
In reality, you have a keyboard matrix, with rows and columns. You have multiple keys attached to each row, and multiple keys attached to each column. However, when you press a key the keyboard gets the "hey, a key is pressed" signal on the (row, column) pair, which the keyboard controller can use to figure out which key you pressed. Therefore, when you press 2 keys in the same row, it won't matter because they're also attached to a different column, so the keyboard can detect that you've pressed 2 unique keys.
But the problem is that even in this matrix format, there are key combinations you can press such that the keyboard can't figure out what keys you have pressed. Modern keyboards actually know when you do this, and they tell the computer to beep to tell you that the computer didn't read your key press.
I just did a search and found a good site that talks about this behavior:
http://www.idevgames.com/articles/programming/id154/And this site links to one that goes into much more depth:
http://www.dribin.org/dave/keyboard/one_html/