by TxRad
You have done all you can with the three of these characters. To continue on with only the three of them at this farm, ranch, or whatever has become boring. Let them expand their fun and games to include other people or, maybe, some sex in public... like, at a swinger's club. This could introduce some insecurity or jealousy might rear its ugly head. Maybe a fun orgy could work. Something, anything else other than the three of them down on the farm. We've been there and done that.
I think introducing others to this story would weaken it. It's strength is in th esweet love of the three of them. A polyamorous relationship is not the same thing as an open relationship.