Generally, when we want to save a file to be worked on another computer, we either save it to a flash drive or to a network drive i.e. Y:/ then browse to the folder where the file shell be save, give the file a name and press save. The file is now ready to be transported via flash drive or opened via network drive.
Sadly 3D Paint projects cannot be moved like traditional way most files can be moved and open on another computer, why Microsoft chose this is a bad idea, flaw, who know but the good news is their is away to move the file from one computer to another one but you will need to do some work on the computer your on now and the computer you plain on working on.
We first need to know where the files are saved, so where are they saved?
Someone from Microsoft’s community form said they are saved in this location “%localappdata%\Packages\Microsoft.MSPaint_8wekyb3d8bbwe\LocalState\Projects“, this is imporant on the computer your working on and the computer you plain on working on.
What is on the “projects” folder?
In side the “projects” folder you will see the following: Projects.json, WorkingFolder, Checkpoint and Checkpoint (#) (Note: The hashtag a.k.a. the number sign repsends a number like one, two, three, ect.
The same thing would apply when you want to email a document to maybe work at home, sent to somebody who is far distance
Unfortunately, if you save a project in 3-D paint, it’s not that actually easy however, after doing a little research and playing around, I managed to find a way that you can do this however, it will require you to do some editing
When you save your 3-D paint project by default, it is saved to this folder
You sent
This particular file is extremely important that we get some information from it so when we arrive at our destination computer, we will either need to drag all of these files over if no projects are here or if there are existing projects, some editing will need to be done
The default location where the files are saved is here %localappdata%\Packages\Microsoft.MSPaint_8wekyb3d8bbwe\LocalState\Projects
Source: https://answers.microsoft.com/en-us/insider/forum/all/paint-3d-default-file-storage-location/e299fa65-7b30-4129-a56e-f698f2c87c58