One other thing is never forget to switch the amp to the 2.67Ω position when using all three cabinets. Forgetting to do so will cause the demise of your amplifier.
In your place I would just leave it at 2.67Ω position even when using just one or two of the cabinets. It may not be quite as loud but I feel it will still be loud enough for most gigs and your amplifier will be safer that way.