To threesome or not to threesome?

My husband and I have been friends for 29 years (met in 9th grade), together for 22 years, and married for 18. Our relationship has been all over the place in that looong time, including on the brink of divorce a couple times. However, this last year we have really finally hit a consistent stride in the best way. We are doing the best we've ever done! No secrets, more time together, more patience and forgiveness, trying new things, etc. For a couple months he's been really vocal about wanting to try a threesome. The third would be his bestfriend (who we are also close to and have been friends with as long). I tend to be the conservative "good girl," so I was pretty taken off guard. He had many partners before me, but he was my first and only partner. I also gained a lot of weight since we first met from autoimmune disease and having kids. My husband definitely doesn't mind, and his friend makes random comments that he clearly doesn't care either. I'm the one with the insecurities, walls, and trepidation. I am curious and the thought is a big turn on, but I'm really nervous for obvious reasons. If you've ever been there, any advice?