It is extreemly important to have the tests. They are blood tests, and you will need genetic counseling for your entire family if your son is positive for fragile X, especially if you have other children or brothers and sisters for thier futures. As for your son's future, if you have a genetic diagnostic you will never be arguing with a school official that your son needs help. Trust me, you will be arguing enough to get him what he needs without having to argue that it is all your fault because you are just a bad parent that he needs special education and targeted interventions, which will be the first line of gate keeping.
There will also be a sense of good prognois. It may not be the prognosis you want, but you will be looking for different aspects for him to progress toward if he has fragile X than if he has a developmental delay with a less predictable prognosis.
Finally, children with Fagile X have a finite set of symptoms but a huge and welcoming support family that you will atomatically find comfort in if your son is diagnosed. You will draw great insight and learn more than you could otherwise about the resources avalaible to your family. You will also know many families that have boys with fragile X, and he will have a true peer group. You have so much more to gain than you can even conceptualize now at age 3. He will never remember this blood test, but he will not forget the knowldge and the insight you gain.
M.