ADVICE PLEASE

I've recently just got out of a three year relationship with my now ex boyfriend. We have both decided to stay friends as we've been in each other's lives for three years! There is one tiny issue!!! I'm sleeping with his best mate (well when I say sleeping I mean like hooking up etc) I really like him and he likes me... my ex has told both of us that we are bad friends is anything happens but I can't help myself, can someone please help me!!!