High maintenance costs and rising unreliability means planes flying to Alderney "must be replaced by the end of the year", it is claimed.
Six trislanders, operated by the Guernsey States-owned airline Aurigny, currently fly from Alderney to Guernsey and Southampton.
Aurigny chief executive Mark Darby said: "The cost of keeping them in the air is now prohibitively expensive."
The purchase of three second-hand Dornier 228 is expected to cost £3m.
Treasury Minister Gavin St Pier has asked the government to guarantee a loan or lend the airline the money.
The airline already has several loans guaranteed by the States, including one which has allowed them to purchase a 122-seat Embraer 195 jet for use on the Gatwick route.
The move follows a two-week trial of the 19-seat Dornier in November.
Five of the 16-seat trislanders could be sold to airlines in South America or Africa, but G-Joey, the airline's pin-up and mascot, could go on display in the island.
There is a lot of affection for the plane, which inspired a series of books and a fan club, with more than 3,500 people on the Save Joey the Trislander Facebook group.
Mr Darby said: "The [trislander] fleet's average age is 37 years and, at £470 per flying hour, their maintenance costs are almost the same as our much larger 72-seat ATR-72 turboprops."
Deputy St Pier said the Dornier would improve the company's finances by £100,000 by cutting operating costs and increasing capacity.
The airline had accumulated losses of £12.5m by the end of 2012 and was expected to make further losses last year.
A review of the aircraft is expected once it is clear what improvements will be made to Alderney airfield under a £100,000 project.
A decision on releasing the money for the purchase or guaranteeing a loan is due to be made in May.