First of all, you are not crazy to want help now. I know you didn't write that you feel that way, but I remember what my son was like at 3, and feeling like what was wrong with the world that no one else could see it or help us. Stick to your guns and intuition and that is the very best thing you can do for your grandson.
That said, I am both the Mom of a 5 yr old with ADHD and co-morbid conditions of Tourette's, ODD (opositional defiant disorder) and Sensory Integration Disorder, as well as the Aunt to a child with Asperger's (on the Autistic spectrum), Tourettes and Sensory Integration. My nephew is 14. Between my sister in law and myself, there is nothing we haven't tried yet, and I've been fortunate to be able to witness first hand some efforts that I didnt' actually have to do the work for.
I can say, witnessing my nephews issues, that the sooner you implement help, the better. A good first step is diet. The Feingold diet, which people will protest hottly as being too difficult or antiquated, has been a life saver for both us and my in-laws. They started it later, so I see how hard it is to get a child to adapt at a later age. We started it loosely at age 3, and it was easier. Food dyes should be illegal as far as I'm concerned, and while the start-up period is a bit tough, it's much easier to follow the diet than it is to deal with the behaviors day in and day out. You can learn more at Feingold.org. They have a book called "why can't my child behave" that is very eye-opening.
At age 3, you will be hard pressed to get a doctor to medicate, which is devastating news, I know. I will say don't waste your time or money on Developmental Pediatricians - they won't help you. Take your grandson to a Pediatric Neurologist and a Psychiatrist. Our son was finally medicated last December at 4 1/2 by his neurogolist - he is on Tenex which is a blood pressure med for adults taht when used in children with ADHD not only helps lower the symptoms, but has no side affects or concerns for liver function or cardiac function like the other meds do. I sobbed the day I medicated him - I felt like I had completely failed as a mother as I sent him on the bus for school that day. When he came home that afternoon, I was amazed. I felt like I met my child for the first time that day, and the combination of diet and meds has been amazing for us.
We also had family counseling through our state funded department of families and children, behavioral unit. It's a division of DDD (Department of Developmental Disabilities). I have to think all states have government programs like these, and DDD in particular is invaluable to hook into for resources. Through this we had 2 family counselors come to the house twice a week, and the difference they made was amazing. It's also free, which as the parent of a special needs kid shelling out thousands of dollars for doctors and help, was a blessed relief.
Lastly, we've just started a biomedical approach based on the book "Healing the New Childhood Epidemics: ADHD, Autism, Allergies and Asthma" By Dr. Kenneth Bock. Reading that book was very eye opening as well and I highly recommend it. It's not the first starting point however - start with the other items, particularly Feingold.
If I can help at all, feel free to contact me. There isn't much we haven't tried, and because of that, the child I had to remove from Pre-school, Music Together and Gymboree and couldn't take into public at all is now going into mainstream kindergarten in fall, and most people cannot believe he has special needs at all. There is hope. It's hard won, but it's there. Hang in there.
Anne