All would certainly put up a CEL and store a trouble code.
I doubt the engine would quit on knock sensor failure - it would just run normally but without knock protection. The engine might quit if either the crank or cam sensor failed, but many engine control systems are redundant to some extent and can get enough information from one sensor to substitute for the other, at least in some "limp home" mode. Some engines don't have both and complete failure of either causes the engine to quit. Or they quit if either fails. So it depends.
not quite mark. there is very little redundancy in the engine design.
knock sensor is for engine protection, but would put the engine into 'limp mode' as mentioned. basically, it would run very rich as to prevent any possibility of detonation and therefore internal damage. all fuel trim would be out the window. prolonged operation in this state would trash the cat.
the crank sensor and cam sensor work together, but the reason there is no redundancy there is due to the fact that the cams turn half the speed of the crank in a 4 cycle engine.
normally, you'd go into limp mode again. if its still running its pretty obvious that its not out of timing. however, it would run terribly and may not even run at all - this depends on the design. i have not dealt with any of these failures in an X3 (my experience with the X3 is quite limited) so it is hard to say.
Whether knock sensor failure would do nothing, simply trigger a trouble code, or cause the engine to go into some kind of limp mode would very much depend on the engine.
Many cars have redundant cam and crankshaft sensor systems. Ideally, you want both operating as the crankshaft sensor gives more precise timing signals while the camshaft sensor can differentiate between the different strokes of the engine. But having no camshaft sensor signal does not preclude the engine running relatively fine as the ignition system can put a waste spark on the alternate cycle without consequence - in fact, many ignition systems are designed to fire the spark plugs at both TDC's. For a non-direct-injection engine, precise timing of the fuel injection is not necessary.
Devil is in the details on this one. Need to know the particular engine to be definitive.