Reply To: How can you resize a group of boxes without changing its gap?

When you resize items like that the spacial relationship would also change slightly, due to increments in scaling.

If you open the Align Panel (SHIFT F7) you can set the space to be 2mm.

Then select all 3 images – hold down CTRL (CMD) SHIFT and drag the 3 images by the corner handle to resize them.

Click the Distribute Horizontal Space to set the space to 2mm.

You'll notice a slight shift in position. Resize the image again, and redistribute the spacing.

But the emboldened part there is probably what you were after, if you want them exactly 2mm apart, then the rest of it is how.