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The third fastener is to ostensibly prevent the standing breech from deflecting backwards under the load of firing. The joint between the action bar and standing breeches is under a giant bending moment when the gun is fired and is a weak spot in the action design. Poorly built guns, or those with incorrect heat treatment, can crack at this location.
The vast majority of third fasteners don't actually make contact in a spot that would allow them to bear load in this axis, so most of them are useless. Exceptions being the Rigby/Bissell rising bite, the Westley Richards bolted doll's head, and the Manufrance Ideal's version of the rising bite. Scott and Greener crossbolts are potentially load bearing but almost never fit properly to do so.
The two locking lug systems are actually just one lug - the bite nearest the hinge pin has such a small moment arm that it's effectively redundant. It serves only to make the gun a snap action by camming the bolt backwards (through means of impingement on the back of the front lug) as the gun shuts.
In general, a gun with a fitted third fastener of the correct design is superior to one without.
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