Need tip!
i have an hot end with 3MM and 3 fillament on 1.75mm.
what can i use as an adapter?
sell 1.75mm filament on ebay and use $$ to buy 3mm filament, or buy a 1.75mm hot-end.
If you’re hot end is 3mm you should be able to use the 1.7mm just fine. You will have to tweak your extrusion rate a bit.
I once put a PFTE-tube with outside diameter 3mm and inside diameter 2mm inside my hot end. This worked for me.
thenk you @Muccaneer_von_Munche “PFTE” was the thing i never heard about, but I have seen and wonder where i can buy it! 
do you know the max temperature of it?
@Patrik_enerstrand PFTE is Polytetrafluoroethylene usually known as Teflon. You find those tubes on ebay or in stores that sell laboratory equipment. Maybe you even find one with an inner diameter 1.8mm . Regarding the temperature - check wikipedia. I only know it is still solid up to 250°C - that is what I tested.
Ensure that the tube is somehow fixed in your hotend that a retraction does not pull it out.
@Patrik_enerstrand if you live in the states, McMaster-Carr is the usual go to place for that type of stuff.
Above 250C it lets off dangerous fumes so be careful
@Eric_Moy ok, no i live in sweden Stockholm
I think it’s PTFE instead of PFTE, if you’re having trouble getting search results with the latter.
@Tony_Olivo … indeed. Sorry for mixing that up.