I bought this 3-gallon compressor a couple of months ago - will it be suitable for performing leak-down tests "in the field" using this leakdown tester from Eastwood? The compressor produces a maximum of 100 psi.

You are set. Thats a cool little compressor. I just did one (compression test) recently using a compressor that was twin 2 gal cylinders and I never came close to running out of air.

Great. That little compressor's turning out to pretty useful.

Actually, a 12V car compressor and a 6 gallon portable tank would do the trick in a pinch. My 12V compressor is good to 240psi and the tank is good to 125psi. You typically want to run leakdown at 100psi because it's easy to get the results in percentage, ie 94 psi = 94%