Guitar Hero 3 was a perfectly fine follow-up to GH2, which was still the best one in the series. It was only when they went to World Tour (GH4) that things started to suck, and even then the gameplay was fine; it was mostly just the menu stuffs that were horrid.
The only reason I can think of why someone would single out GH3 and say it sucked was because RB1 came out at the same time, which cut into the audience of Guitar Hero players considerably. People who play for fun would naturally prefer Rock Band, whereas people who played for challenge would prefer Guitar Hero 3.