You say it fired 3 and the fourth did not go off. Sounds more like dollar box ammo to me. if you attempted to fire several more and it dod not fire I would worry.
That is what is called a mossberg hammerless, but the hammeerless seies are the only ones that have a hammer. Figure that out. The firing pin just sticks out the rear of the bolt. as long it is free and the front not broken off it should work.
another thing about that model is it cocks the hammer on pulling the bolt to the rear. If it is hard to pull. remove the stock you will see a rod with a spring around it going from the hammer back, this goes through a little bar that hooks into the bracket for the trigger. Put some grease on the rod and it will work wonders.
also if you remove the bolt don;t pull the trigger the hammer has to cocked to get the bolt in.
Hope it gives you alot of shooting.