See Transferring files for a comprehensive set of instructions for both Android and iOS.
You can export your files from v2.9.x as a PSD format from iTunes or Dropbox and open them in SketchBook v3.0.
1. From the Gallery, select your canvas and Export from v2.9.x to Dropbox - select Layered PSD format.
2. Install the Dropbox app and launch Dropbox
3. Select your file and Open In..
4. Select Open in SketchBook
5. The selected canvas image should open in SketchBook v3.0
1. From the Gallery, select your canvas and Export from v2.9.x to iTunes - select Layered PSD format.
2. Connect your device to iTunes and save the file from v2.9.x to your desktop (or any other file location).
3. Add the saved file to v3.0
4. Select the file
5. The file is added to SketchBook Documents
6. Import the image via iTunes in v3.0
7. Your canvas image will open in SketchBook v3.0