Take out two and weigh them on both sides of the balance (for the first time)
Situation 1: The balance is balanced.
Then the eight balls are all normal, and the special ones are four.
Take out three of the remaining four balls and put them aside, and put three normal balls on the other side (the second time)
If it is balanced, what is special is the remaining one.
If it is not balanced, it is in three boxes above the balance. And know whether it is heavy or light.
Take two of the remaining three and weigh them. Because we already know the weight, we can know the special one. (the third time)
Case 2: Balance tilt.
The special ball is at one eighth of the balance.
Write the four balls on the heavy side as A 1A2A3A4, and the light ones as B 1B2B3B4.
The rest are determined to be four normal, marked C.
Put A 1B2B3B4 aside, and B 1 and three normal c balls aside. (second time)
Situation 1: The balance is balanced.
The special ball is in A2A3A4, and I know that the special ball is heavier.
Weigh A2A3 and you will know which of the three is more special. (the third time)
Case 2: the balance is still A 1, which is relatively heavy.
The special ball is between A 1 and B 1.
Just take an ordinary scale and you will know which one is special. (the third time)
Situation 3: On the other hand, the balance is heavier at B 1
The special ball is in the middle of B2B3B4, and it is known that the special ball is lighter.
Weigh B2B3 and you'll know which one is special. (the third time)