Given the challenges of parenting a child with Autism, it is understandable that you would be concerned about the possibility of your two other son's "getting Autism." As you know, Autism is characterized by a number of debilitating symptoms, including significant impairments in social interaction, language deficits, intense interests in focused topics, and sometimes, arm flapping.
Unfortunately, however, researchers are still learning about the causes and consequences of Autism. It is generally accepted that Autism has a large genetic component (family relatedness) and there is no concrete evidence that different environmental factors (people or things in the child's life) contribute to the development of Autism. Therefore, there is little that you can "do" to prevent the onset of Autism, but as you may have found, children with Autism generally exhibit symptoms before age 3. That is, children eventually diagnosed with Autism are described by their parents as being "different" very early on. So, the best thing you can do for your children is advocate for early intervention services for your child diagnosed with Autism and be the best parent that you can be (firm, consistent, and loving parenting) with all of your children.
Autism is genetic. Either your boys have the gene or they don't. There are many new vitamen therapies however that seem very promising in treating autism and the earlier it is discovered the better so don't be afraid to be a little paranoid and ask the dr about every symptom your boys display.
But all your child will want to know why there are differences between the siblings. I would suggest first learning more about autism yourself, and then educate all your children about the diagnosis.
There are several great books out there, including My Friend with Autism (for younger kids) and the Autism Acceptance Book (for older Elementary and middle school students)