One of the most frequently asked questions we receive is: What size filament should I use in my Ultimaker?
The Ultimaker family of printers uses 2.85mm filament, which is commonly called 3mm filament to differentiate it from the 1.75mm filament used by some other machines. Originally all "3mm" filament was 2.85mm, but as more companies have begun making it, many of them were unaware that what was called 3mm traditionally had specs of 2.85mm, +/- .05mm. When purchasing filament, please check the specifications to ensure what you're viewing is actually 2.85mm.
Can I use filament that's 3mm wide, or do I need to use 2.85mm filament?
You can print with 3mm filament, but you may need to make some modifications, and there are no guarantees about print quality since you'd be switching to some non-Ultimaker parts. Any issues arising from use of non-Ultimaker parts or modification of existing parts is not covered under the warranty. True 3mm filament is probably struggling to get through is the Bowden tube and Teflon coupler. The interior of the Bowden tube should have a 1/8 inch opening, which is around 3.1mm. Since both the tube and the filament may also have a little variation, it will probably be a little too tight a fit. You may also be experiencing a tight fit in the Teflon coupler as well for the same reason.
You can look for a Bowden tube with a larger diameter, though there is no official larger diameter bowden tube from Ultimaker. You might get slightly worse retraction performance with a wider tube, but it should allow the filament to pass through. You might also want to check your Teflon coupler and possibly drill that out fractionally wider to allow the filament to pass if you're using 3mm filament.
You will also need to go into the Materials settings on your machine and adjust the filament diameter in the menu, so that the printer can calculate the correct amount of plastic to push through.