Actually, DD isn't 3yo until Saturday.... she speaks much more poorly than other children we know. Quite hard to understand, although within the family we can usually work it out. The only consistent thing I can take note of is that she doesn't say S or Z sounds within a word. So she can say "See", but ice cream comes out "Ah Heam", please comes out "Peeee", her own name (isabel) comes out "ih-ha-bel".
Does it matter? All the other little ones we know close to her in age can say S and Z fine.