Welcome!!! 3 is the age my daughter was diagnosed too. The out of sync child I think is a great book for understanding SPD but it doesn't give you much in terms of what to DO other than "see an occupational therapist" I think OT is great but if I were to make a list of things that helped my daughter, it would probably come in below a lot of other things, such as:
A diet free of gluten, casein, soy and artificial food coloring
Supplements such as cod liver oil, probiotics, b-12 spray, calcium, vitamin D and zinc (zinc helped big time with picky eating)
Taking baths at night with epsom salts in them
Doing daily activities that involve crossing the midline of her body to help organize her nervous system
Buying a Waldorf i.e. nature inspired homeschooling curriculum to to with her at home (we use Oak Meadow)
Participating regularly in a multi age playgroup through our homeschooling organization (emphasis on multi age)

We have also done cranial sacral therapy, but I would rank that just below OT in terms of helpfulness, which is not to say it wasn't helpful, just not as big of a wow factor in the differences it made.

I really do not think any of the things I have done would have helped much if we hadn't of done her special diet. I think observing children's art can be a very useful window into their minds and hearts. Here are two pictures my daughter drew of herself the spring before she turned 4, 1 is right before she started her diet, the other is just 6 weeks later.

One thing I have found is that although possibly an autism diagnosis is not appropriate for our children, many of the things that help children with autism, also help them. I would not be afraid to read and research things relating to Autism, I especially like Dr. Sear's new Autism book.