Your lead screw is 3mm you need a 1.66666666666666666666666 mm pitch thread, so that's a ratio of 1.8:1 - i.e. 1.8 turns of spindle to one turn of the lead screw. To avoid any maths just swap the ratios into teeth e.g. a 10Tgear on the lead screw would require an 18T on the lead screw. 10T & 18T are too small, so multiply both by say 3 and you get 30T and 54T
i.e by pass the box, if you can, and use 30T driving a 54T (or any similar combination) . If you can't bypass the box, set it to 1.5mm and use 30T driving 28T ( or 60:54 etc.)
Check pitch before cutting ;)
Bill