Every car you see in my signature line uses DOT 3.
DOT 3 will hold up just fine, even if the car isn't driven very often. Everything has to be sealed up tightly so no moisture gets into the lines.
DOT 5 doesn't always play nice with rubber seals and hoses and may disintegrate rubber seals in your old wheel cylinders. It's also more prone to foaming which will give you a soft pedal.
Unless you've replaced every single component in your system, there will likely still be DOT 3 in the system which doesn't mix with DOT 5. ERvena small amount of DOT 3 will cause serious problems in a DOT 5 system - gunked up lines, sticky mess, etc.
