Need advice

Hope

So my fiance and me have been talking about having a threesome for a few months now, we've decided a few rules and stuff for either gender but were both straight and he really wants me to eat another girl out and I'm open to the idea but I'm honestly scared and I dont wanna chicken out last minute and him think differently of me. Plus we are both the jealous type so it's a big thing of I dont want him to think shes better than me. I just dont know what to do, and I dont want it to ruin our relationship by bringing someone else into it guy or girl..