One thing id be a little concerned about (even at this price range) regarding the PM3 are materials used. $399 is the same price as the B&O H6 which leather, lambskin, aluminium and only a SMALL amount of plastic. If the PM2 lost all the quality materials likely as a reason to justify a price gap from the PM1 Im guessing the PM3 will nearly be all plastic. IMO if its something that can be worn in public then it should be look as good as it sounds.
Have you actually seen the PM-1 / PM-2 / PM-3 all side by side? ...or felt the difference between materials?
They are pretty much indistinguishable from one another when it comes to look and feel of the materials.
I think the PM-3 look stunning to the naked eye when viewed from bystander perspective. I haven't heard them yet myself, but that will be rectified soon enough.
Also, Oppo has not said there was any compromise to quality of construction between the PM-2 and the PM-3. That is more or less your assumption based on price.
There are other differences between the headphone that can account for a lower price, like different/smaller drivers and ear cups, as well as permanent pads as opposed to removable ones, and the single sided cable connection instead of dual sided.
They will also most likely not come with a long, high quality 6.35 mm connection cable like the PM-1/2 does, since the PM-3 if being sold primarily as a portable headphone.