Step 1: Analyze statement I.
Ordering the numbers from highest to lowest and selecting the middle number is equivalent to finding the median, not the average. Therefore, statement I is not equivalent to the average.
Step 2: Analyze statement II.
Dividing each number by 3 and then adding the results is the same as adding the numbers together first and then dividing the sum by 3. This is equivalent to calculating the average. Hence, statement II is correct.
Step 3: Analyze statement III.
Multiplying each number by 6 and then dividing the sum by 9 does not give the same result as dividing the sum by 3. This is not equivalent to finding the average, so statement III is incorrect.
Conclusion:
Thus, only statement II is correct, and the correct answer is (B) II only.