My son did the same thing! He has an older brother and was speaking extremely well at a very young age, he could sing his whole ABC's before he turned two and was speaking full sentences when he was 2, people were amazed at how well he could talk!
But then all of a sudden he started to stutter, I did have a new baby so I think that had a huge part in it. I think he was maybe wanting to sound more babyish and cutesy, but he would even stutter when he cried, so I was confused! My pediatrician told me not to worry and it's probably just a phase.
The phase did pass after a few weeks and he forgot about the stuttering. I just tried to ignore it when he did it, that's what I read to do, to not focus on the stuttering when they do it.
I do think that when kids hit three, even if they don't have a younger sibling, they start to realize that they're not much of a baby anymore and they are turning into a "big kid." I think they still do want to be babied a little bit, so they kind of revert back.
I'm sure it's just a phase, good luck!