Asking my SO for a threesome?

So i consider myself the kinkier one in our relationship, always suggesting new things to do or toys to try out, but idk how he'll react if I ask him about having a threesome. 😳 More and more often lately I find myself wanting to experiment with another woman or another guy with us. He's very traditional and protective of me, so how would I go about suggesting this to him?? Also how would we even find someone to have a threesome with? 😂