I am installing the anniversary update for windows 10 as I type this. When it is done, I shall check the store for the pc version as Microsoft stated it would be there. I shall keep you guys posted with what I find.
Well, anniversary update installed, no forza horizon 3 in the windows store. In the Xbox app it is not there either (under windows games). One can find forza horizon 3 under Xbox one games still, with a little difference. The purchasing services are down at the moment so maybe we need to wait for a bit. Fingers crossed, I shall keep you posted.