Thank you for reaching out, I will try and give you some advice but, please note that I am just a mere woman with my own opinions and this is not to be taken as gospel.
Before bringing someone into your love making sessions to spice things up, I would just ask yourselves one thing first, Is my marriage 100% solid? If your answer is yes and both of you aren’t going to be jealous of the each other then, if thats what you both want and your both completely sure, then why not. I would recommend that this shouldn’t be a regular action because there will come a time when one of you wants to continue and the other doesn’t. Set some guidelines for yourselves and maybe a safe word that only the two of you know about. If you’ve both been completely honest with each other then this shouldn’t hurt your marriage.
I think most definitely the third person should be a stranger. If you bring someone you know into a 3some then your opening the door for lot’s of problems that could arise down the road. i.e familiarity, suggestive remarks and maybe difficult meets in the future. A stranger would eliminate those worries. Then there’s other things to think about. Both your husband and yourself would have to choose wisely and I would recommend meeting up with this person before taking the plunge. It’s a very intimate thing you’re both venturing into and it would help if this person gives off the right vibes for you both. From my experience, most people lie about themselves online hence arranging to meet up first.
Safety is always paramount so always make sure you have plenty of condoms and use them. Don’t drink too much on your first encounter, you will need to aware of your surroundings and don’t take valuables with you i.e lots of money, expensive watches or phones. I would suggest buying a cheap burner phone with a pay as you go sim to contact your wise choice. This will give you complete anonymity to safe guard your family if things go pear shaped.
I hope my answers have been helpful, don’t rush into anything, take your time and then the both of you should enjoy a spicy session of your choosing.
Good luck to you both and have fun!