I'm not aware of any funding to top up your monthly stipend; though I assume your university makes some provision for travel and research costs through smaller schemes. (You'll need to apply for these on a case-by-case basis.)
Beyond that, try to get some teaching work and save this money for the inevitable spill over into a fourth year, at which point I assume your funding will stop. (I'm not suggesting you can't submit within three years, though you'd be the exception rather than the rule.)