The reason we don't use girth strap 2 and 3 together on a normal saddle is that they are attatched to the tree by the same peice of webbing and if it snaps, both straps come undone. The first strap is attatched with a different peice of webbing.
However on a cob saddle there are four straps. The first one is a point strap and is bolted onto the tree points. The second and third are 'normal' girth straps and may or may not be the same peice of webbing. The last strap is a 'balance' strap and is anchored much further back than normal straps.
The different straps can be used to secure the saddle the right distance down the back. The girth always wants to make the small circumfrence around the horse so the saddle is pulled either forwards or back so the saddle sits directly over the girthing center.
That means with a pony with who has no withers, big shoulders and forwards girth groove (i.e the girth sits right under the elbow) the saddle is dragged forwards by the girth until the saddle sits right over the girth and the saddle is half way up the neck

It's a very common problem.
Looks like this (my fabulously photoshopped horse

)
So you would want to use the first (point) and second girth strap so the girth pull is as reduced as possible.
So it's more like this
However, if the saddle slides back (common on hunter types) you would want to use the back straps so the girth pulls the saddle forwards.
And on 'normal' horses you would use the middle two.
The point strap does drag the points of the tree down onto the back though and therefore the only time you would use it is if your saddles is being dragged forwards. The balancing strap is suppose the stop the back of the saddle rolling but doesn't usually work.