The simplest things can amuse a three-year-old. My son loves to look at the computer while I work on it, and three things are his absolute favorites:
The screen saver. We have a screen saver with fish on it, and he loves to point out the sting ray and the shark, and tell me the colors of the various fish, over and over again.
Alison Kent's website. He adores her website because it has little circular buttons. He loves to watch the buttons come up one by one and chants, "One-two-fwee-four-five-six" as they come up. He demands "the circles" about thirty times a day.
My website. Specifically, he loves the cover of NEVER LOVE A STRANGER. He takes great joy in pointing at the nude guy on the cover, laughing maniacally, and yelling, "He gots no cwothes on! THAT'S CWAZY!!"