We report a novel type of monolithically integrated tunable semiconductor laser. The tuning is achieved by three intra-cavity Mach-Zehnder interferometers, realised in passive waveguides and using voltage controlled electro-optic phase modulators requiring only four control voltages. The potential of the design is demonstrated by a realised laser system that shows an optical linewidth of 363 kHz, output power of 3 mW and a record tuning range of 74.3 nm. Such a continuous wavelength span is in excess of any monolithic semiconductor laser reported up to date. Precision of the tuning mechanism is demonstrated by a scan over a 0.89 GHz wide absorption line of acetylene. The laser design is suitable for a number of applications including gas spectroscopy, telecommunication and optical coherence tomography. The laser has been fabricated in a Multi-Project Wafer run on an indium phosphide based, generic photonic foundry platform and demonstrates the potential of these technology platforms.