Conversion of 3mm ABS filament to 1.75mm
Photo Credit: Courtesy of 3DStockPhoto ( image)

The best option is to find somebody in need of 3mm filament and trade them for it (either in exchange for 1.75mm 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.