A Flybe flight from Dundee and KLM Air France flight from Amsterdam landed at Manchester and a KLM Air France flight from Lyon landed at East Midlands.
Most flights have been able to land at the airport.
The Instrument Landing System (ILS) has now been fixed and there are no delays to flights, the airport said.
A spokesman for the airport said the computer system went down at about 0910 GMT.
He said the three aircraft were diverted either because of airline procedures or because the pilots were not trained sufficiently to deal with heavy fog without the ILS system.