I think if no arguing went on in friendships, they wouldn't be real.
It seems to me like you three are so close, you are like family. Families argue.
Personally I think you three need to get together and sit down to talk about what's happening. If you've been friends for a while, you might just be drifting away. Friendship groups change rapidly over time, and what you do about it will effect the outcome; whether it survives or not. One thing is certain, if the arguing and backstabbing is bad, and you don't take action, you might lose your best friends.

If you are talking to one of them and they say something bad about the other one, remind them that you're supposed to accept each other as friends, and not talk about each other negatively. Tell them you don't like that the backstabbing is happening, and if they react in a way that won't help the situation at all, they're obviously not mature enough to continue the friendship group without arguments.
Hopefully it's not that serious.. and it works out for you.
Good luck.