Conversation so far:
When did Tom Petty join the Traveling Wilburys?
In 1988, Tom Petty joined George Harrison's group, the Traveling Wilburys.
What did Tom Petty do in the Traveling Wilburys?
For the Traveling Wilburys' Traveling Wilburys Vol. 3. album, Bob Dylan and Tom Petty became lead vocalists as George Harrison ceded the role to them.

Question: Who else was in the group?
Self-contained rewrite: Who else was in the Traveling Wilburys besides Tom Petty, George Harrison, and Bob Dylan?