I had a niece and a nephew who both had lisps and problems speaking clearly when they were that age and the cause was actually quite simple. Both were still using their pacifier a lot. They were walking around for most of the day with the pacifier in their mouth and learned to speak "around' it. When the pacifier was taken away from them, their speech greatly improved, although lisps are still slightly evident. Check to see if this is the problem first before spending a lot of time and money on a doctor. If it's not the case, they may need some speech therapy, but 3 yrs old is still pretty young. If he started speaking later than average, then this might be part of the problem. Not all kids develop at the same rate. There are also certain things that might effect his speech such as a large tongue, a small mouth or a high roof of the mouth. All of these can cause difficulty speaking clearly.
As far as the vocabulary, again not all children develop at the same rate. If he is still using his pacifier, then he might be limiting his words and sentences because he either has a hard time speaking with the pacifier in, or just doesn't like having to take it out of his mouth for long periods of time to speak. This is exactly what my nephew did. Just keep reading to them and encourage them to read. This is the best way to expand their vocabulary and if you encourage them to read aloud to you, you can help them pronounce their words by gently correcting them when need be. My nephew's grandmother would have him read books to her and she would help him with the words. Needless to say, his speech and vocabulary have greatly improved.
Good luck!