Well for me, there just wasn’t quite enough space between the pommel and my horses withers. I will eventually have my saddle properly fitted so that no shims are needed. The ultra thinline shim, 3/16" thickness, provided just that extra little bit of space over the withers for my peace of mind.
I would assume you would know that you need the middle shim if the saddle was bridging, that’s when the saddle makes contact at the front and back but not in the middle, common with sway back horses.
And the back ones you would use if the saddle appeared to be too high in front, tipping too low in back
I’m by no means a saddle fitter so if you’re unsure I would ask a professional. Sometimes if you take pictures of the saddle on the horses back and send them to a saddle fitter they will be willing to give you some free basic advice, but make sure you contact them and ask first 