I am surprised they only have a 3 year warranty - other brands seem to have a 5, 7 or 10 year warranty.
There's usually a longer warranty if you have it fitted by an Approved Installer. I remember mine had a 10-year non-corrosion guarantee on the heat exchanger, but as it's stainless steel, you'd expect it to last almost indefinitely. There's not so much inside them to go wrong.
edit, sorry, too slow.