Anxiety Issues in 3 year old

I am lurking here. I have a 9 year old with Sensory issues (I tend to stick to the hypotonia board for some reason...) and anxiety. I think that Caren gave great suggestions. My son was anxious in many of the same ways your son is (transitions were killer). Finally in 2nd grade we ended up in a psychiatrist's office for attention issues. (there is a long story on how we got there but we will cut to the chase) He was diagnosed with anxiety and the dr put him on the lowest dose of prozac. I was freaked out...never thought it would work (I was thinking that ritalin was where we were heading). It was night and day as far as living with this child. He was still his sweet self but the edge was gone. The walking on eggshells feeling with living with him...so reduced. It was a miracle. He is happier. He is more focused at school (still has sensory issues but they are reduced). We are calmer in our home. I wanted to add that as a possible option (he is so young yet, I know) that maybe your path will lead you there. I am so thankful someone was able to see that it was necessary on top of many of the other tools Caren suggested.