Welcome to the forum and I kinda feel a little for you in your plight with the customer.
Hate to say it, but the description you have given is nowhere near enough information to offer an informed decision.
Just for a tiny start, you have not mentioned...
Lateral loads from any direction.
Bracing of the fixed "legs." (Tube overlap will count for squat if legs are not braced to carry lateral, rotational, axial or shock loads)
Gotta say also, I would be against air for such a weight (THREE TONS). Air is compressible and Hydraulic fluid is not. It would be nuts (in my opinion) to proceed with air just because the customer does not want hydraulics. Who pays when the air system turns out to be to unreliable and even dangerous after x-thousands of dollars have been spent getting to that point?
At this stage I will not get to the other questions as you need (in my opinion) to stand up to the customer and tell them they need an Engineer to design the system and then you can build it.
If the customer balks then you would do well to recommend your nearest and greatest industry-competitor to take on the job.
Just one old fart's, been-there-seen-that-didn't-like-it opinion.