I've thought about it frequently since then, especially when my partner and I come across that elusive third whose sexual compatibility with us enables him to slip between our sheets quite comfortably. The rules about adding a permanent third to an existing relationship are, in fact, not at all different from those that apply to a more traditional relationship. Honesty is key. Open and honest discussion is key. This alleviates any possibility of sexual tension and jealousy.
Problems are sure to follow.
The point is, both of you have to really want a third. This would seem obvious, but the tendency to accept a third that doesn't entirely thrill you in order to make your partner happy is a powerful one. Both of you have to have equal say in the choice of a third. No one enjoys feeling like an outsider, especially in bed. Never assume your third is looking for a long-term commitment.