Hi Sue,
These are three great cars. The Golf is a lot more expensive, new or used. The Seat Leon and Mazda 3 are cheaper new and depreciate slightly more aggressively. So you can afford a 3-year-old Leon or a two year old Mazda.
At the moment, on small diesels, the Leon will depreciate less over the next two years. On larger petrol engines they are about the same.
They are both reliable, but all cars have unique weak spots to check before buying. They will be listed on our website as they come to light.