When my son was three he too had speech difficulties, especially with the letters "C" "ck" sound, and "T"'s. We took him to be evaluated and that's when we realized he was diffrent.
The speech therapists would put photos in front of him and ask him to identify the object and tell them the name. The first object was a candle. He wouldn't say the word, he picked up a crayon and spelled the word out on a piece of paper. This went on for about 15 minutes, until they realized this wasn't a fluke.
Six months later we discovered why he had a speech problem, we are bilingual my husbands side of the family is Italian, and the way my son's gradmother pronounces some English words are very different, and my son was mimicking her:)
I too was worried that my son would be diagnosed with something like Aspberger's because of his lack of talking, however, at almost three he began to speak in full paragraphs and blew us away. The doctor said that he was focusing on learning other things and talking wasn't important to him yet.
My son was obsessed at that age about trains, and a few others things.
Hang in there -- and keep searching for answers and don't rest until you are satisfied.
