A third man is accusing a former Deerfield Academy teacher of sexually abusing him while he was a student there in the 1980s, the accuser’s lawyer said Tuesday. Boston attorney Mitchell Garabedian said he is representing the man and one of the other two former students who say they were abused by teacher Peter Hindle, who is retired. The Western Massachusetts prep school recently said that two of its former students had accused Hindle of sexual misconduct and that he admitted to committing a sex act with one of them. A Deerfield spokesman had no comment Tuesday. The Northwestern district attorney’s office is also investigating the allegations.