The pH affects the activiy of enzymes.
Enzymes are a catalysts. They affect the speed of reaction.
Extreme pHs, very low or very high, regularly drives to a poor performance of the enzymes, even to total loss of activity.
pH 3.0 is a very acid pH and any enzyme will not show activity at that pH.
The optimum range of pH for Amylase is 6.7 to 7.0, that is why she found that the reaction was fastest at pH 6.7.