I have a few kg of 3 mm filament when I only have use for 1.75 mm.
How can I make 1.75 mm from 3 mm filament?
In theory, making filament is easy. You take a 3 mm hotend with a 1.75 mm hole, and extrude the 3 mm (sometimes actually 2.85 mm) filament, let it cool, and then reel it up.
In reality there are a lot of pitfalls to making filament - if the pressure isn't even, the hole not perfect, the temperature uneven, you can end up with oval filament, filament with bubbles, or worse. If you are over temperature you may damage the filament and it could look good, but not melt correctly when used. If you reel it too fast you may thin it out more than the intended diameter, or too slow and you may thicken it. A lot of hotends use steppers, which may result in ripples in the filament, so you may end up building a nearly custom filament machine.
Resolving all these problems is probably not worth simply selling or giving away the filament to someone that can use it, and buying the right size for your machine.
If you are still interested, though, you might as well go all the way and build a full filament extruder that accepts raw plastic feedstock (usually pellets) as well as your filament, and convert it that way, then continue using it to create your own filament.
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.
You could build a machine that has a nozzle with 3 mm input hole and 1.75 mm output hole, based on some designs for filament making machines. Or you could just cut the filament into little peaces and use them instead of the granulate in an original filament making machines.
However, as mentioned by kaine, this is very unlikely to be worth the cost/effort. Best option for you is to try to sell the 3 mm filament to someone who has a use for it, take the money and buy some 1.75 mm filament instead.