It looks like a nice saddle, I have no idea about the price since I'm not too up on the market.
However, the easy change gullet system changes the GULLET of the saddle ONLY! There is no change in the tree. It is meant to, and is pretty good at, make small adjustments to adjust/perfect the fit around the withers on a saddle that already fits the horse fairly well. I think most of these come with regular/medium trees and that does not change. You can put an extra wide gullet in it and you still have an extra wide gullet on a MEDIUM tree. As a consequence these saddles simply DO NOT FIT some horses. Also as a consequence if you have 3 very different horses that NEED different tree sizes you will still NEED different saddles because the gullet system does not change the tree only makes relatively minor adjustments around the withers.
On the other hand if your horses are fairly close in size this could work well, but then if they are fairly close in size you can probably also get a standard saddle that fits them all pretty well and adjust with padding.
Another thing to think about with this saddle is that it's wool and it's used. Wool is thought to be a good thing because it molds really well to the horse and it can be restuffed. BUT this saddle is used, so it's already molded to at least one horse before which WILL affect the fit on your horses (you see this with foam too to some extent). You may find you need to reflock the saddle even if the tree is a good fit (not horrible, but you should think about that in the pricing/your budget). Also the benefits of wool won't be seen as much if you are continually swapping the saddle around from horse to horse as it won't mold like it's designed to do.
There are saddles that supposedly adjust in the tree area as well (specifically wintec's air ride "CAIR" pannels and some of the treeless/flex tree saddles that I don't have much knowledge of). However, I know at least wintec recommends their CAIR system be set up by a professional saddle fitter to your horse and then left alone unless your horse changes do to muscling, etc. It's not something you want to be fiddling with daily to make it fit multiple horses, especially if they are vastly different in shape.