A Somerville man authorities say was a leading member of New York’s Colombo crime family tried for the third time to plead guilty Thursday, but a transformer fire forced postponement of the proceeding. Ralph DeLeo, 69, had just admitted conspiring with others to run a drug ring that sold cocaine and marijuana when the hearing came to an abrupt end. The lights went out briefly and fire alarms sounded, forcing the evacuation of the Moakley Federal Courthouse.