Our 3 yo does the same thing, I wouldn't worry. He seems to do it when he's really exicted to tell us something and just can't get it out fast enough, so he ends up stuttering because his mind is working faster than his little brain.
I understand your concern my daughter did the same thing around that age and I was so worried that I had her evaluated by a speech therapist, who told me that it was completly normal and that it would pass as she got a little older and it did with time it totally went away she is almost 5 now and has no problems with speech at all. I would say its most likely a phase but keep an eye on it as mommy you know your child better than anyone, if its still a concern later down the road I would speak to your childs Dr.
REally - don't worry - HE DOES have a lot to say and needs to slow down a little - Also I noticed my grandson stuttering at that age - because it seemed what he had to say was important for him to - and he was afraid to be interrupted.