I never saw a 3x tele-converter. I only learned about their existance from as test in a US photo-magazine from ca. 1970 where the 3x ended worst.
But if one needs that effect that moment on the lens one got, the technical quality of the image is of lesser interest.
I have to say, I am rather impressed with the results.
Most of the reviews I read were negative for the 3x, but not the 2x for the reasons stated by posters above.
I realize this person tested with a d***. But, I like to tinker about a bit with my gadgets too so, needless to say, I will play with my old beat-up 3x TC over the holidays.