I'm going to be cold towards my friends given the circumstances which they started dating each other under
Don't you think that's a little bit rude? What lead to them dating each other?
I was fucking them at the same time
That makes sense that you would be hurt in this situation, but their relationship is their choice and not your business. Maybe you should try talking to them about how you feel instead of being cold.
It's not up to me. They knew I was fucking both of them and they still did it.
I feel like you're not considering their emotions when you were hooking up with them at the same time, but right now that they have done something that affected you, you are not happy about it. I think your issue with your friends can be resolved by talking sincerely with one another and making sure that you tell all of your issues with them and I am sure they will tell theirs as well.