Only sum may cross #2

Five minutes later Harlow solved the problem. When the six balls were correctly placed in the holes, the gate swung open. Harlow scurried across. He did not trust trolls. Soon he came to the second bridge. It also had a gate made of iron bars that blocked the way across and a padlock. The second padlock had eight holes laid out in the shape of a square.

This time eight clay balls were lying on the ground in a row. The first ball had 1 marked on it; the second had 2 on it, the third had 3, the fourth had 4, the fifth had 5, and the sixth ball had 6, the seventh ball had 7; and the eighth ball had 8 marked on it. There was a message tacked to the gate. It read:

The gate will open to those who can add to twelve. Put one ball in each hole in a special way. The sum of the three numbers that are on each side of the square must always equal 12.

Harlow was worried but he began to think anyway.