Are you sure it is actually the buttons that have failed?
I had button #3 on my TM joystick (pinky switch) fail after about three years. It was actually showing "on" all the time.
I took it apart, removed the button and found it was functioning correctly. I even replaced the button and got the same fault. I took the stick apart further to check for rubbed or pinched wires but found all was good. I therefore narrowed it down to either the PCB in the joystick grip (less likely) or the IC mounted on that PCB (more likely IMHO).
I put it down to either IC failure over time, or possibly the result of occasional static shocks I received when I initially touched the stick from time to time. I'm just guessing on this though.
Luckily, being button #3, it was easy enough to reassign those functions to another button with a modifier. With three buttons gone though, that is not really a workable solution for you.
Hopefully it is just faulty buttons, which is a fairly easy fix. However, with three buttons gone (at the same time (?) and all showing "on," my money is unfortunately on something a little more sinister. Hope you get it sorted.