Most likely it could run on the x3 (I don't see any limitations on the SoC itself, might be wrong) but it probably wouldn't be a good experience. Remember what happened when they tried to put W10 onto the older Lumias? It ran, sure, but the experience wasn't good so they didn't want to roll it out publically.
There's a lot more to getting a desktop OS (even a slimmer variant like W10M) running on tiny devices than just processor architecture (which wasn't an issue with the older Lumias, and probably won't be an issue now). You could totally run Win10 AU on a device with a comparable CPU and 3GB of RAM, but once you factor in the heat and power limitations of a very thin, 6" device, it's not going to be a great experience, and might not be worthwhile to pursue since there's only so much you can optimize before it becomes more practical to switch out the bottlenecked component.
(Of course, maybe I'm wrong; I didn't think it was possible to get x86 code running on an ARM chip, but here we are.)
I'd personally love to see them do it anyways though. Would be great to say that all these devices already out in user's hands can now run full Windows - it's not like they promised it would be a good experience, or even existed at all, so there's really no downside there. But I wouldn't expect them to go out of their way to get it working.