The 3 amigos problem can come from the modulator, or the wheel sensors. When it's coming form the modulator, it is usually brake fluid seeping past the o-rings that are inside the ABS modulator and that are preventing the valve switches from getting contaminated. Once they are contaminated you get the 3 amigos. You can either replace the modulator with a new one, or a used one that will eventually present the same problem, or buy a kit to replace the switches and o-rings. Once you have the modulator apart, it's a good idea to bypass a connector that is inside and subject to the same contamination, by a two pole universal connector.

Those repairs, in the Discovery circles in various forums are known as the A fix and the B fix. Google A B fix 3 amigos, and you'll find how to definitely get rid of that problem, for around a hundred dollar if memory serves. In my humble opinion, I doubt that "exercising" the shutle valves in the case of the Disco 2 modulator will make a difference because when I took mine apart the brake fluid was clear and there were not visible traces of varnish or some sort of dirt that could prevent the modulator from working correctly. But I am not a mechanic, just a hobbyist.