I pre-ordered my FP3 when I read on the fairphone site that they want to bring out an open OS for FP3, too. Got my FP3 finally in the beginning of November. Since then I am waiting for the open OS. I feel a bit cheated now to hear there will be no FP3 OS.
I’m no dev, so I can’t support working forward to an open OS for FP3.
But there are some articles concerning open source licencing in the present c’t (german IT journal). They say if the Android Open Source Project is based on GPL and FP3 OS is based on Android FP3 OS kernel sources must be released for FP3 customers.