I should preface this feature with a comment or two on there not being a version of Paint 3D for mobile - or at least not yet. In fairness, the UI for Paint 3D is (relatively) complicated enough as it is, in terms of controls, sliders and so on - it would be a genuinely impressive undertaking to get the application just as useable on a 5.2" phone screen. Could it be done? Possibly - after all, Office Mobile is exemplary here, but I doubt that getting Paint 3D reworked to auto-handle phone displays is a priority for Microsoft. And after all, creating 3D models is really a desktop/studio thing, you're not going to be knocking anything non-trivial up on the bus home from work on your phone!
Having a viewer for 3D files does make sense on the phone though - this way, a friend could email over (or share via OneDrive, etc.) a model they've been working on and you can spin it around and see what you think. All part of Microsoft's Creators Update world - and quite impressive at that.
Just to show the workflow, I'll make a trivial model in Paint 3D on my Windows 10 Creators Update-running Surface Pro, save it to my OneDrive, and then open it up in View 3D on my Lumia 950 XL, before re-uploading it to the Remix 3D model sharing site (run by Microsoft). It's easy!
The workflow is surprisingly seamless, considering that built-in 3D modelling on a consumer OS is very new. You have to think that a small team in Microsoft is even now figuring out ways to optimise Paint 3D for mobile too - with the touch interface it could be very workable, though perhaps only on 5.7"+ screens, so think phablets like the 950 XL and Elite x3.
Comments welcome if you've played with any of this!