Peter Pan: If he's out, he's usually outside Pirates of the Caribbean. He also sometimes appears in Town Square at MK and at MGM either near the Sorcerer's Hat or at the park entrance. He's also a regular on the Family Magic Tour (but never guaranteed).
Timon and Pumbaa: Pumbaa does not appear in the parks for meet-and-greets, but Timon is found in a lot of places. I seem to find him at Town Square at MK, on Character Bus at EPCOT, and at the Hat at MGM the most.
Ariel: Ariel appears at Ariel's Grotto in Fantasyland most of the time. If the temperature is bellow freezing, she is moved to Cinderella's room at Toontown Hall of Fame. She's also there when Ariel's Grotto is under rehab.
If you have any trouble finding them, ask at City Hall at MK or Guest Relations at EPCOT, MGM, or AK. They can usually call a hotline and tell you where and when a character is scheduled to appear. Most character escorts are also knowledgeable about character whereabouts as well.