Hi ctc,
Tested in Numbers 5.2 under macOS 10.13.6 High Sierra. (I can't remember how far back previous versions were able to do this).
First, apply these settings in Numbers > Preferences. Tick every box. You will glad that you did.

Second, go to Print Setup (command p) to see the paper size.
I am using A4 paper in Portrait orientation.

210 x 297 mm with 2 centimetre margins.
If you are using another paper size, such as US Letter, you will need to use inches.
Here we go with A4 paper and millimetres (which become centimetres on Numbers rulers 😕).

Formula in the top table cell D2 =A2−B2−C2
Formula in the bottom table cell D2 =A2−B2−C2
Now insert a rectangle Shape. White fill, black border, to make it easy to select and see.

With the Rectangle selected, Format Panel > Arrange > Size (and make the size just a little bit less than the print area, perhaps the width of a bee's knees).

Print Setup (command p)

Now you have a rectangle to show where you can place your table inside a page.
And don't forget about those Alignment Guides. They will help you to place a table in the middle of the rectangle.
Save as Template for future use!
Regards,
Ian.