Officials of the Central Canada Exhibition have canceled a three-performance singing engagement by Anita Bryant in August because of threats by homosexuals, her Canadian agent, Ken Campbell, said. in Toronto. Miss Bryant led a campaign against a homosexual rights bill in Dade County, Fla., in 1977 and later lost some singing engagements in the United States be cause promoters felt she had become too controversial. Campbell did not specify the nature of the threats and exhibition officials have refused to comment.