“Toy Story 3” begins with a rattling, exuberant set piece that has nothing to do with the tale that follows but that nonetheless sums up the ingenuity, and some of the paradoxes, that have made this Pixar franchise so marvelous and so successful. The major toys — Woody (Tom Hanks), Buzz (Tim Allen), the Potato Heads (Don Rickles and Estelle Harris), Hamm (John Ratzenberger), Jessie (Joan Cusack), Rex (Wallace Shawn) and the others — are in a setting at once wholly unfamiliar and instantly recognizable. They’re in a western, albeit one made in the amped-up modern action style, rather than…