Construction of a third MX beamline at the Australian Synchrotron has been approved with the project starting in July 2019. The science drivers for the new MX3 beamline are: small weakly diffracting crystals, serial crystallography and new sample delivery methods. The MX user community is advancing the field and working on larger proteins and complexes that are producing smaller, more weakly diffracting crystals. The increase in membrane protein crystallography, especially GPCRs, is driving the need for a smaller beam than MX2 with a higher flux. The beamline will complement the two existing MX beamlines (and the new ADS beamline) and will provide a smaller beam size with higher flux than is available on the current MX2 microfocus beamline. Serial crystallography (fixed-target and injectors) will be a new key capability. MX3 will be placed on sector 04, with a 3m u17.5mm hybrid undulator as a source. High flux will be produced through the use of a water-cooled horizontal double multilayer monochromator (DMM) with three stripes, 0.25%, 0.5% and 1% bandpass. Modelling of the optics predicts a full beam at the sample of 8.5 x 2 microns (HxV, FWHM) with a flux of 1.1e14 (ph/s/0.1%bw). Closing the secondary source aperture to 35 microns will produce a 2x2 micron beam with a flux of 2e13 (ph/s/0.1%bw). The beam can be defocused via moving the sample position downstream in the endstation to at least 20x10 microns. The endstation will contain a large area detector (such as an Eiger-16M), a high precision goniometer (MD3-UP) with 50nm sphere of confusion and a fast sample loading robot (capable of loading pins and trays onto the goniometer). The goniometer will provide pin-compatible serial crystallography and a separate fixed-target scanning system will provide chip-based scanning. The goniometer will be capable of in-tray screening and collection. Injector systems will also be available. Systems for in-tray screening will be provided to crystallisation facilities to assist users in improving crystals. The beamline will provide a high degree of automation including automated raster-and-collect for micro crystals, automated scanning of chips and plates and automated data reduction and merging.