just a hint, I've 3D modelled and printed quite a bit. If you print a simple cylinder with an inner dimension of 2,5 mm, you can just take an old M3 fine thread screw, file two grooves in it in the length of the screw, and you have made your own 'tap'. As it just needs to cut plastic, it'll work. Partly it will cut, partly it will push a thread in.