DIAGNOSIS Air compressors have a huge startup surge—often 3 to 5 times their running wattage. Your 3500-watt generator can handle about 4375 watts surge, but a big compressor can easily exceed that if other things are already running.
FIX You need to manage the startup load. First, make sure the air compressor's tank is completely drained and the pressure switch is set to OFF, so the motor doesn't kick in immediately. Turn everything else on the generator OFF. Start the generator. Plug in the compressor alone and let it run until the tank is full and the motor cycles off. Now you have a buffer. With the compressor motor off, you can then plug in and start your other tools or appliances one by one, staying under the 3500-watt running limit. The compressor will now only cycle on occasionally, and its surge will be less likely to trip the overload.
IF THAT DIDN'T WORK If the compressor alone trips the overload on startup, it's simply too big for the generator. Check the compressor's label for Locked Rotor Amps (LRA) or startup watts. You might need a smaller compressor or a "soft-start" device for the compressor motor.
HEADS UP Repeatedly tripping the overload can stress the inverter components over time. It's better to undershoot your load than to constantly ride the line and trip it. Consider the running watts of everything, and leave a 20% buffer for surge.