This kind of approach invites trouble .
Development of the Fairphone 3 is at a very early stage, there are no real specifications or anything to go on apart from the basic direction given in the latest interview. How do you know that this new phone will fit your needs?
But given the final specs are out and you like them … what if availability gets seriously delayed … or Fairphone change the announcement to 2019? Such things happen all the time.
But given it really is available let’s say Q4 2018 … what if your needs or use cases changed until then?
If you are in need of a phone now, then consider the options available now.
If you are in need of a phone later, I think you could at least wait until Fairphone flesh out the final specifications of the next phone to get an idea of whether it fits your projected needs, but don’t take 2018 for granted.