Brake fluid tends to absorb moisture

So after long usage, the absorbed water may cause sponge when braking. However, it is hard to tell when you should flush the brake fluid. 3 years is not very short but not too long. If you are in area with a lot of raining days, and the brake fluid reservior cap has been openned many times, you may want to consider it. The reality for me was that my car in place like MA for 4-5 years but never flushed. The reason is that typically when replacing the brake pad, portion of the fluid will be removed and replaced with fresh one, and that will slow down the moisturing process. However, not all shop will remove the fluid as indicated by SOP.
So my comment will be, you may want to consider it, but only if you feel sponge when braking. If you plan to go to mountain area, flush it before you go is a good idea. DO NOT go to dealer to do it.