In the Gimp entry fields for numbers, you can enter simple formulas. So you don't need to compute
1200*3 manually on the side, you just enter
1200*3 in the input field, and Gimp will set the field to 3600.
A special case of this is of course to do
Image > Canvas size and add a
*3 at the end of the horizontal/vertical size fields.
Replicating/tiling a picture
If the picture is under 1000*1000px:
- Create/open the target image, set its canvas size if necessary, add a layer if necessary, etc...
- In the "tile" picture, do
Select > All,
Edit > Copy to copy the picture to the clipboard
- In the Bucket-fill tool, set the options to:
- Fill whole selection
- Pattern fill
- Click the pattern icon and select the "Clipboard" pattern (this is always the first in the list, so the top left corner in the mosaic of patterns)
Instead of creating a new image you can of course change the canvas size of the current one, copy the existing layer to the clipboard, add a new layer, make sure it is as big as the canvas, and then bucket-fill.