Chevrolet: Camshaft/Crankshaft sensors, driving in the rain, domino effect

Question
I have a 97 Venture with a 3.4L v6. The SERVICE ENGINE SOON light has been on for a few days and the error codes report that voltages coming from the camshaft and (2)crankshaft sensors are out of range (P0336 and P0341).

My mechanic wants to simply replace all three sensors.

It'll cost over $500 and makes me wonder if he's diagnosed it properly.  Could THREE sensors fail in some sort of domino effect?  Are they treating a symptom rather than the underlying cause?

Any advice would be appreciated.  Thanks.

Answer
He wants to replace all three sensors because he doesn't want to test them to see which particular one has failed. Sensors failing are like playing slot machines. Usually you get craps but once in a blue moon you might get two or three that fail at once but its highly unlikely.

In this case it would be just the two sensors not three. Also I have to ask, did the light come on after a heavy rain fall or driving in the rain?