Yes. Wire on Teensy3 supports slave mode.
Early in Teensy3's history, I made several revisions to the Wire library port. The slave mode support was the last thing added, many months after Teensy3's initial release. In fact, as I recall, for a while one of the handlers was supported but the other wasn't. If you search old messages, there's probably a lot of conversation from those early days. Eventually, I did get both working. I have personally tested both, so I can confirm they do work.
I believe there is one minor quirk. On Teensy 2.0 and Arduino, the handler is called while the AVR still holds the I2C bus busy. The SDA and SCL lines are allowed to return high only after your handler function returns. On Teensy 3.0, your handler function gets called while the bus is idle, right after those lines return high. I'm not sure if that would ever really matter. But unfortunately, that behavior together with Wire's API that allows variable length reception (you don't have to tell Wire in advance how many bytes the other I2C master sends) just isn't possible on Teensy3.