Will the object maintain one orientation? If so, you only need the value of the vertically oriented accelerometer. If not, then you need to take the vector length as per your math. Will it be lifted evenly by a machine, or will it be all over the shop (lifted by a human)?

When an object is being lifted, at the start of the lift the total acceleration in the object is somewhat greater than g. At the end of the lift, the acceleration is less than g - you can experience this in an elevator. If you want to to work that way, then you don't want to be subtracting the average acceleration from the sensed acceleration. If you subtract the average acceleration, you will be able to sense the top and the bottom of the lift, but you won't know which is which (because mathematically, a start and a stop are both simply accelerations).

The difficulty is that at the start of a drop, the total is less than g and at the end of a drop the total is greater than g. So to sense a lift, you need to sense a >g followed by a <g. Furthermore, if the object is being lifted unevenly you can get spurious events.

You could try using acelleration to plot an absolute position … but the data will be noisy and inaccurate and it's really not going to work.

I would approach this by logging the data from the acellerometers during a series of lifts, dumping the data into excel, and seeing if I can formulate some sort of rule in excel that correctly senses a lift. Then I'd translate that rule into a sketch.