I just came across a really good theory on Tumblr, saying how Donald and Goofy help Sora to stay positive--because of their promise to each other to "keep smiling"--and also pick up on a lot of things that he necessarily doesn't.
Or, at the very least, they might have a different opinion or view on something, that then makes Sora stop for a second to actually consider things (as seen in Chain of Memories).
The theory then goes on to say that maybe because Donald and Goofy weren't with Sora in Dream Drop Distance to help better him--and that this was also his adventure alone--that maybe that's why he fell into Xehanort's trap so easily and got consumed by the darkness in it.
So in KHIII, maybe Sora's learned his lesson and that's why he thinks it's best to bring them along with him again?
(That, and I'm pretty sure that Donald and Goofy are raring to go on an adventure again with Sora, because of their bond, as well as to be able to "do something important"--as said in DDD.)