@layden.lee234,
There is no special tool for this. You need to do the manually.
Footprints are usually snapped to the centre of their pads which is not very helpful for adjusting edge to edge spacing.
An easy way to do it might be to create a 3mm wide track on the Document Layer and then move that around to use it as your guide to the size of the gap between the silkscreen outlines of components.
You may need to change the snap size (turn snap off) to move the guide and the Footprints to give an exact 3mm gap.
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