When can I bye a Fairfone3???
I am not sure, if even Fairphone can tell you a certain date.
They just anounced - a while ago - that there will be a new phone-model later this year.
From previous experience I would be prepared to wait til spring 2020. While there might be a bit more pressure now to shorten the “phoneless time”, the main target sure is to present a phone that is fully developed and thoroughly tested.
On a side note … while the topic title makes the intention clear, for avoiding possible future misunderstandings please consider https://brians.wsu.edu/2016/05/19/by-bye-buy/ .
(Misunderstanding example translating to German at DeepL)