Yes, 3D printers do work in micro gravity. At least if they're designed to do so. The 3D printer from Made In Space was sent up to the ISS to be tested.
And it's made it's first object - a part for, well, itself. The faceplate with the Made In Space and NASA logos will help hold wiring in place on the printhead.
Now, astronauts will be able to use the printer to manufacture small parts and tools. This is absolutely key. Instead of having to send up every part the ISS might need, all Earth has to send now is the design (which is digital) and a suitable amount of raw materials.
The next part of the test will be to print an object and see how it compares to an identical one made on Earth, to see just how well the extruder is actually working compared to one running in normal gravity.