Our son was diagnosed earlier than three. However he has been in weekly speech therapy sessions since he was 18 months of age. At the beginning it was very much an elimination process, where we started a program similar to what you would use for an autistic child (based on reward etc) this obviously did not work, as he could not form any sounds. Over time the diagnosis became very clear. He is now 3.6years old and he is a text-book posterboy for CAS.
The good news is, with the right direction and regular speech therapy we have come so far! we have lots of words and sentences. The longer the sentence or the word, the muddled it all does become though. We have used the nuffield program which has worked best for us. The use of hand cues has meant that rather than sitting down for formal "homework" we practise throughout the day, emphasizing and highlighting the different sounds as we speak.
I will also add, that Isaac did see an OT once a month for a year. Her focus was concentrating on Isaacs "sitting muscles". His OT was fantastic at putting my mind at ease that there was nothing further to worry about. An additional bonus was that her own son has CAS.
Goodluck with your journey, sometimes it seems so unfair and feels like you aren't getting anywhere - but soon those days will be behind you.