If I've got a 3-box hive, can I use the Flow Super Lifter to lift the top box and inspect the middle box?

Yes, you can use the Flow Super Lifter to lift both the upper box and the middle box on a three-box hive. See the user guide for more info.

The maximum height from the ground to the base of the (deep) box underneath the one you're lifting is 70cm / 27.5 inches.