Here is a breakdown of each statement:

I. "the acceleration is zero when the bob passes through the mean position." This statement is correct. At the mean position, the pendulum has maximum speed and is moving in a straight line for an instant with no force acting along its path, hence no acceleration.

II. "in each cycle the bob attains a given velocity twice." This is also correct. In one complete oscillation, the bob passes through the mean position twice - once moving in each direction - hence it attains the same speed twice.

III. "both acceleration and velocity of the bob are zero when it reaches its extreme position during its oscillation." This is incorrect. At the extreme position, the bob has zero velocity as it momentarily stops before reversing direction. However, at this point the acceleration is at maximum as it is the force which is pulling the bob back towards the mean position.

IV. "the amplitude of oscillation of the simple pendulum decreases with time." This is also correct in a real-world scenario due to air resistance and internal friction. However, in an ideal, frictionless scenario, the amplitude would stay constant.

So option 3 (I, II, and IV) is the correct answer.