2d "printed" antennas are actually common and in commercial use. For instance, what appears to be flat panel antennas on cell towers are sometimes arrays of what appear to be J-poles including phasing sections on a PCB in a case. Bluetooth and wifi antennas embedded in devices are commonly printed directly on the PCB for the device.
PCB antennas actually work quite well, but are difficult to make, as standard PCB has variable dielectric, so a higher quality PCB is needed. When modeling the antenna, the dielectric constant of the PCB has to be taken into account, along with any surrounding case.
3d printing itself is can be quite useful for making antennas, but printing the metal in the antenna is unlikely to be useful as the conductivity of the result is likely to be poor and/or variable and the surface of the print may be too rough. More useful would be to print jigs, forms, and mounting hardware for the antenna. There are many examples of this on thingiverse.com that can be searched for. Complex antennas contain both conductive and non-conductive parts, and 3d printed non-conductive parts can be useful, although again, dielectric constant of the plastic may affect the antenna, depending on placement and size of the parts and the variable density of the parts.
RF in antennas largely travels on the outside of parts, so it might be interesting to 3d print forms for the antenna and then wrap the forms in copper foil or adhesive tape. Thickness of the foil may be important for transmitting power carrying capacity. If the metal is thick enough, the dielectric of the plastic on the inside will not have much effect.