Wow, fascinating story! Thanks for sharing it. I was struck by the statements regarding the benefits of having one life partner/lover as a support when the third one dies, a benefit that two-paired individuals don't have. It must be very powerful to have two lovers mourning the loss of the third one together and supporting each other through this process.
Our culture tends to disapprove and/or pathologize relationship arrangements it does not fully understand or does not fit a rigidly pre-set socially-accepted relationship paradigm. Nevertheless this testimony exposes us to the possibility that yes, alternative relationship arrangements can work as long as there is open and honest communication between individuals that share common values and expectations.