As
@neilly has already said, the best, actually the only way to difinitively find out what's going on is a code reader. Be careful not to use one of the cheapy ones off the bay, they MIGHT give you results, but there's no way to know whether or not they're telling the truth.
There are probably five "common" faults which can cause the 3 Amigos, and some less common ones too. The first four out of the five are the wheel speed sensors, one on each wheel, guessing can be expensive. The fifth reason is the Shuttle Valve Switch plate.
Under normal conditions I would suggest, even recommend that you carry out the well known modification to the SVS plate. It often costs nothing, if you don't replace the plate, about 40 quid if you do, it requires a couple of bits of wire and an hour or two to accomplish. At least, it would take that cause out of the "equation" for good.
Google "Disco 2 SVS modification" or search on the forum here for more details.
If you look at the "landroverclubvi.weebly.com/abs-mod.html" page, it will tell you all sorts of stuff about removing and rebuilding the ABS modulator, DO NOT get dispondent, scroll to the bottom of the page, all you need to do really is "Option B".