First, evaluate the given function at x=3 to find the amount of bacteria left after 3 seconds:

There are 750 bacteria left after 3 seconds. Since the initial amount of bacteria was 6000, subtract 750 from 6000 to find the amount of bacteria that has been removed:

Therefore, the amount of bacteria that has been removed after 3 seconds is 5250.
The correct choice is Option D) 5250.