The reason their are 3 girth straps on most saddles, is in the good old hunting days, one may have broken so you would have another as a back up. Now days people often use the 1st and 3nd girth strap to balance out the weight of the saddle, and for safety.
err I though the girth straps were attached differently ie first and second attached into saddle on one link and the third and a separate link - in case one link into the body of the saddle breaks.
Often you use the first and third strap as they are attached differently, but it depends upon the make/design of the saddle. So you need to look and see how they are attached.
My dressage saddle only has two straps, but the jumping one has three and for safety I use the first and third.
and just to confuse matters...my cob synthetic has 4!!
i use 1st+3rd (or would use 2nd and 4th)
look how the leather is attached to the saddle,it should be on 2 different pieces if webbing/canvas,so use 1 from each webbing and the other is for emergency breakages,i think.
You should always use the first and third strap. This is for safety reasons, as the first strap is single, and attaches to and ends at the tree, it does not cross over the saddle to the strap on the other side. whillst the second and third are joined both continue over the saddle giving a little more security. Should a strap break, the spare (middle) strap will hold enough to get you safely home.
If you were to use a the first and second, or second and third, in normal use your saddle would not be held in place correctly, and could move ie. lift at the back etc.
I have used last 2 in the past if horse has an elbow rub but my saddle is noticeably less stable that way so I would only ever do slowish stuff and be very careful but it just means I can still ride and try and keep the weight off him
I agree with what LD says about conformation of most saddles
The first strap is sewn to it's own piece of webbing which goes from one side to the other - this is the one that should ALWAYS be used.
The 2nd & 3rd straps are sewn onto the same piece of webbing - because of this you never use the two together - if the webbing breaks then your saddle comes off!
Using the third strap does help to sit the saddle firmly on the horses back.
They used to be if a strap broke sio you had a spare back in the old days and with some saddles it's still the case.
Although many saddles these days are now are designed so you can use any to help balance the saddle, it's important that your straps are situated in the natural girthing line, otherwise your saddle will pull forwards or back, although this really it is down to the fit of your saddle ultimately but having the option also helps.
Cahill - If you have a 4th strap, attached to the back, then this strap is to help the saddle stay still on the horses back. My coloured native has a flat back and a round rib cage and the saddle slipped sideways so a balance strap was added to the saddle. I used the 1st and 4th strap and saddle was more stable. (A prolite non-slip pad is also extremely useful!)