Can i trut my best mate and boyfriend?

It all started 2 yrs ago me, my best mate and partner had a 3 some and after that they asked if i would do it again, but i wouldnt.. Anyways she is suppose to be coming over this weekend to stay the night and my partner is picking her up and wont let me go and i dont think i trust them.. THEY BOTH SAID NOTHING WILL HAPPEN. Why would they say that?. But then they both asked me on different days about doing 3's again and i said NO, now they both have hump with me but i dont know why.. This is why i am so worried that something will go on behind my back when he picks her up or if i get too drunk... Please help.. Should i trust them and let it go? I have asked if they have feeling for each other but both denie it.. I need ure help!!!