Allegis Corp.'s third-quarter profit dropped 25% to $82.3 million, due in part to a one-time gain in year-ago period, while profit at its United Airlines unit shot up 159%. Chicago-based Allegis said revenue totaled $2.26 billion, compared to $1.96 billion a year ago. The third-quarter profit drop was aggravated by the one-time, pretax gain of $81 million Allegis received in third quarter 1986 through the sale of two hotels. Allegis said the airline had a profit of $84 million in the third quarter, despite a 38% increase in jet fuel costs, compared to $48 million a year ago. United's operating revenue of $2.3 billion was 15% ahead of last year's quarter. Allegis Chairman Frank Olson attributed United's strength to improved service. Allegis sold its Hilton International chain to Britain's Ladbroke PLC in September and announced earlier this month that it would sell Hertz Corp. to a new company partly owned by Ford Motor Co.