First, there is this statement from Fairphone:
https://support.fairphone.com/hc/en-us/articles/360032971751-Operating-systems-OS-for-the-Fairphone-3
Then you might want to check this thread:
It’s not about PureOS, but the information will be valid for your question as well.
No Open OS for the moment. And the rest might depend on the community (which is a quite active one).