<p>I have a few kg of 3&nbsp;mm filament when I only have use for 1.75&nbsp;mm.</p>

<p>How can I make 1.75&nbsp;mm from 3&nbsp;mm filament?</p>

The best option is to find somebody in need of 3 mm filament and trade them for it (either in exchange for 1.75 mm filament or in exchange for legal tender with which to buy said filament).
The next best option would be to cut it into small pieces, and feed those into a filament extrusion system such as the
filastruder
.