Amplifier Transfer Function Question
The author is talking about small-signal gain, not large-signal. A small change in the input of 0.01V will change the output by about 0.03V. Although it's actually a negative gain here, as an increase in the input means a decrease in the output.

This is an estimate, though; just visually looking at the graph I would estimate a gain more around -2.5 or perhaps a bit more, probably lower magnitude than the 3 stated in the text.