Somebody knew you'd take care of them. Only two look like they may have been from the same litter so they may have been rescued but the person didn't know what to do with them afterwards. One is definitely Lab and the other two are wonderful mixes.
God bless you for taking them in and if you put them on Petfinder you may find them a good home if you don't want to keep them.
I have a friend that lives out in the country and her place if a favorite drop for unwanted pets. She keeps and cares for them all.
Labmomma (Donna)