According to the meaning of the question, the three containers are filled with the same weight of water, not the same weight of sugar water, that is, the weight of each sugar water is different after putting sugar.

Let the water weight of each container be 1, then

The first container: the weight of sugar is 2/3, and the weight of sugar water is 1+2/3=5/3.

The second container: the proportion of sugar and syrup is 3: 7, so the ratio of sugar to water is 3/4, the weight of sugar is 3/4, and the weight of syrup is 7/4.

The third container: the ratio of sugar water to water is 9: 5, so the sugar water ratio is 4/5, the sugar weight is 4/5 and the sugar water weight is 9/5.

Mix together: sugar weight 2/3+3/4+4/5= 133/60.

The weight of sugar water is 5/3+7/4+9/5=3 13/60.

The concentration of sugar water is (133/60)/(313/60) ×100%.

=42.5%