I have recently been pointed in the direction of Fairphone, but one detail appears to be missing from the Technical Specification that I regard as important: the “flavour” of Android.
I am a great proponent of Android One, as it has less BloatWare than most OEM versions, and is updated faster than Android versions that are either Manufacturer and/or Carrier specific.
Fairphone would seem philosophically well aligned to Android One, but there is no definitive statement as to whether FP3 is and/or will be using this, just some speculation on this Forum.
For a definitive statement I would contact Fairphone directly, because as you correctly noted, people here can only make (educated) guesses since this is a community forum.
Regarding your question: FP is currently not using Android One, but near-stock Android Pie, which also hardly contains bloatware apart from G00gle apps. I haven’t looked into Android One very much and while I like the concept of long-term updates, I also fear that using Andoid One would give G00gle even more power over the operating system and could make a G00gle-free OS by Fairphone themselve or the community very difficult to realise, therefore I am not in favour of it.