It's my opinion that they were given the title because a great teacher produces great students (as said by Jiraiya in the anime). The 3rd was the greatest teacher for them to have at the time. Of course that's not the only reason, they obviously had lots of skill to be training but they were all geniuses in thier own ways. They all had drive too, they wanted to become better so they took the time to do so. I think that they wanted to become better because they saw what they could become (the 3rd) with training. A great teacher really does make all the difference. They did fight together, you have to go through all those missions before the chuunin exam then the chuunin exam together so I'm guessing that all three of them also excelled in those test a lot.

This is just my opinion, feel free to add whatever you think.