When you're in Tamahara, the Age in which the game starts, go to Atrus' desk. Hold the [~] key and click on the picture of Atrus and Catherine on Atrus' desk. A screen will show showing the actors who played the characters in the game.
When you're in Saavedro's home, go near the hammock. When you zoom in, hold the [~] key and move your cursor around. You should see a zoom-in cursor. If this is the right one, release the [~] key and it should disappear. Hold the [~] key and zoom in. You will find a PEZ dispenser, with the initials MEB on it, for Mike Brown, Myst III's lead animator, who has a lot of PEZ dispensers.
J'nanin Age: Lens posts
On the J'nanin Age, there are a variety of posts, each with a sort of lens. You must find the source of light (near the yellow post) and turn the cover until the light is directed to the yellow post. From there, point the light from post to post and ultimately to the receptor next to one of the three tusks. The light posts can be manipulated by clicking on the ring under the lens. Record the order of the colors of the posts that the light travels to in order to reach the receptor, this order is the key to open the door.