It turns out I can successfully trigger the Pinch With Thumb And 3 Fingers gesture maybe one time out of 50-150 attempts (it's a rough estimate because the sample size of successful attempts is really small). This same rate of success seems to hold both when I'm using the Magic Trackpad while the MacBook is in clamshell mode, and when I'm using the MacBook's internal trackpad while the laptop's lid is open, the Magic Trackpad turned off, and the external display disconnected.
I don't notice any difference between how I perform the gesture on the successful attempts and the unsuccessful ones - it just seems to randomly work sometimes. Weird.