I would think that you would be able to answer this better than us since you know them.
My advice is split the prize pool and have fun.
no friend, I discovered that they were friends and play together in tournaments at the time.
Friends play against each other in tournaments all the time. They are usually trying to beat each other more than they are trying to beat you. Unless you see them colluding or acting suspiciously, just play your game. Unless you can prove they are cheating, you just have to go about your own business and try to win.
Pay attention but they would have to be really good at colluding with only 3 players for the dealer and floor to keep track of. If you are worried verbalize your knowledge with them and go from there.