as i understand it, and i may be wrong (again) dot 3 brake fluid is hydroscopic and can/does absorb H2o effectively diluting it throughout the entire brake hydraulic system. If H2o finds its way in (and it will) dot 3 can handle a considerable amount of moisture (relatively speaking) without it severely affecting its performance, and by diluting it, the corrosiveness is greatly reduced. Dot 5 on the other hand, isn't as friendly to H2o. Water can and does find its way in and accumulates in condensed ratios causing very focused corrosion of the brake system components.

I know I've been bumped on the head many times and may be way off so check for yourselves, as for me, I will be using Dot 5 in my classics when possible, just don't rely on it lasting forever on cars that don't see much drive time regularly.
"You get what you pay for"