For any type of SHTF you would not need a shotgun to kill birds just shoot them in a tree or on the ground with a 22. Walking around hunting will be a waste of time. It can also kill people and is quit so you could shoot people from even as far away as 150m and they would have no way of knowing where you are for at least a while. So with that my first choice would be a
Ruger 10/22 with a 4 power scope
The next gun would for some kind of SD. So for this I would go with a rifle not carbine. M1a with USGI parts and an Acog will kill any game in America and it is a great combat weapon. The noise it makes alone will make people piss themselves. Also one very important aspect a .308 rd goes through cover and will still kill people; 5.56 gets deflecting to easily so go big 1 bullet does a lot of damage so you can carry less. My choice for second gun would be
Springfield M1A with USGI parts and ACOG
For my third choice most people would think some kind of hand gun or shotgun but those aren't really that important for me. Handguns aren't good hunters and they are very limited in combat use. Why waste the weight of a hand gun when you can carry more ammo for your rifle. Both of my previous choices can get me food and defend my self with. So unless I have a third person with me I would just carry some more ammo. If I had a third person depending on who it was it would be another M1a or if it was my wife whe would carry a AR15 with none of the extra stuff on it. It is easier for her to handle then an M1A as she doesn't have long arms. Any more then 2 guns for one person and you are just wasting weight. there is a lot of other things that you could carry. Just to name a few warm cloths(MTNs get cold), food or maybe even some water.
My point is Guns are great and they are very cool but lets be practical you can only shoot one at a time might as well carry less variants and carry supplies instead.
Just to add I have way to many guns cause they are that great!