Okay. If I were you getting your first saddle, I would get it fitted by a professional, and they can talk you through the process. But if you can't do that then here a few guidelines:
- To fit the saddle, make sure it is clearing his shoulder blades either side, so it may be a bit further back than you are used to. A lot of riding schools/trainer teach beginners to put the saddle right on the wither, which is better than too far back, but not correct for every horse.
- Always check the fit of the saddle without a saddle pad.
- You're correct, about three fingers clearance over the wither is good. It also shouldn't sit to high off the wither, as this will put too much pressure either side of the spine, so working off the three fingers Idea is good.
- If you look down the gullet (the space in the middle that sits over the spine) you should be able to see daylight through the other end. Liek you said, no part of the saddle should be touching the spine.
- Have a look at the saddle from the side. It should be balanced, the pommel and cantle sitting at the same height, neither one higher than the other, and the base of the seat should be the lowest point.
- walk your horse around with the saddle on, girth undone and no saddle pad. The saddle should not slide forward or back, and when turning should still sit reasonably square. It should not lift up at the back or the front.
- If all of the above are correct, girth him up and take him for a short ride with no saddle pad, just enough to make him sweat under the saddle. Sitting in the saddle you should feel centered, not tipping forward or back, and with a line from your head through your shoulder and hip to your heel. When you take the saddle off, have a look at the sweat patches. There should be no dry patches, or patches where the hair is disturbed. If the sweat is even, then cogratulations, your saddle is as well fitted as you can get it yourself.
Whenever I get a saddle, I roughly fit it myself, and then asap I get a saddle fitter out just to check it over. It may be that it needs more/less flocking, sometimes the gullet isn't the only thing that needs to be changed for a good fit, however you shouldn't have that problem with a new saddle.
Also, Wintec sell a kind of hinge that is used to measure what size gullet your horse will need. I think they cost about $30 here, and it saves you the trouble of buying a gullet and it not fitting.