Talk talk talk talk about everything beforehand. Is every on the table with you, her, and the guest star? Can everyone kiss/lick/cum wherever anyone wants? Protection? Can you finish in your guest or does your wife want to be the finale? What happens afterwards? Are you all okay with cuddling all together? Will you be upset if she just wants to cuddle with your wife? Vice versa? Make sure the guest knows any boundaries you decide on before they arrive.

Also note, if any of these things cause a raise of anxiety or severe worry maybe step back before acting on the threesome. Took months of conversation before me and my husband were ready and had everything on the table.