Powering a 230v air compressor

This could easily get sticky, too late, and not enough details

1.........Too late now, I guess you could have just bought an engine powered compressor. It takes a HUGE generator to start a big 240V motor. Most generators "lie" about capabilities, meaning that if it says it will start and run a 3 hp motor, that probably means 1 1/2--2 instead, and certainly not on a long run of undersized wiring.

2.........Depending on where the fuse box is located, consider having a 240V outlet installed BY SOMEONE QUALIFIED right at the box or nearby. Then make up an extension cord to reach the garage, for temporary use. This will have to be heavy enough wire to carry the load

4.........Tapping into the AC. This is something I would and have done, but not knowing your skill, or the attitude of the landlord, that would be WAY down the list, and if you are not skilled with "juice" we might be reading about you later. Seriously dead, that is.