Not that it is important, but sin(3x) would have 3 times the frequency of sin(x) and an octave is just double the frequency. I do like the 3 times louder part of your explanation.
Not to be picky but loudness is logarithmic with amplitude...
Darn, I screwed that one up. you're right. good call! and thanks for the correction; and it's important to me anyway :)