I disagree with your pediatrician, especially for a 3 year old boy. My experience is that some docs will just tell you what they think you want to hear. If you were to ask them what they would do if it were their child, I would bet the answer would be different.
Both my sons had issues with some speech oddities: mis-pronounced letters and some annunciation issues, even documented by an evaluation at school. However, around 7, my older son's issues were entirely gone and my younger son's resolved at around 10, but he is a very creative child and he can still revert to the "odd" annunciation (i.e., his "baby" voice). However, if you are worried, you should use what's available through the school system. My concern is that kids are very perceptive and IMHO this sends a very subtle message that there's something wrong with your son, when there very likely is not and per my experience with my younger son, it may suppress his creativity as he's trained that there is one "right" way to speak - he may later be afraid of being creative with his voice.