Separate the one file into three files, one for each page. When you've done that, open the file and choose Camera>Standard Views>Top
The choose Camera>Parallel Projection
Next, set the background color as white
To do this go to Windows>Styles and hit the "Edit" tab at the top
Next, hit the third box over, right under the currently selected "Edit" tab. This will bring up a box like this:
Uncheck the box that says "sky" and click on the colored square next to "Background"
That will bring up another box. Set the third slider at 100 and the second slider at 0, so it looks like this:
Close out these boxes.
Next, go to File>Export>Export 2D Graphic
Enter a file name and hit "Options"
Enter a suitably high resolution-I usually do 6500 pixels in width. Mess around and see what works best for you. I like to do it very hi-res.
Once you've done with that, hit "Export" and come back in five minutes. It can take a while. After that, import the complete file into GIMP and edit.
Let me know how this works for you, and if you have any questions, feel free to ask.
