The man police say is the prime suspect in the slayings of five college students in Gainesville, Fla., was convicted of burglary. It was the third conviction for Danny Rolling stemming from a Tampa, Fla., crime spree that occurred days after the students were found stabbed to death in their apartments last year. A jury convicted Rolling in the Sept. 2, 1990, home burglary, which occurred about 1 1/2 hours after he robbed a grocery store of $3,000 at gunpoint. No charges have been filed in the student killings. Prosecutors plan to present evidence to a grand jury in November against Rolling and former University of Florida student Edward Humphrey. The burglary and petty theft convictions could result in a 30-year prison sentence for Rolling. He already has been sentenced to life in prison for another grocery store robbery.