Otherwise known as hygroscopic.
Depends what sort of driving you do, but it's always good practise to maintain them especially if you've spent alot of money on the braking setup. ATE superblue recommends you change the fluid every 3 years but mine don't last half that long if I fit in alot of track days, for that reason the driver should be aware of when his/her brake fluids aren't performing.
If you can feel a sponginess (assuming it's been bled correctly) and a lack of progressiveness as a result of absorption of water in the brake fluid then it's just logical to change the brake fluid regardless of how long the fluid has been in the system for. I certainly would not wait till the 3 years knowing full well my brake fluid has absorbed a signficant amount of water and reduced it's boiling point as a result affecting the braking performance and response lol
The 3 years is a rough guideline of it's lifetime on average use, as deemed average by the company - average use could be anything within reason. It is not to be followed religiously, as many will have you believe, certainly like gearbox oil where manufacturers claim it need never be changed, ever, because it's designed for the lifetime of the vehicle but we all know that to not be true.
