I haven't seen my son in over three years and I'm not really sure on how to go about seeing him after such a long period?

How old is the child.

Assuming the child is older and will understand, be honest, don't make excuses and don't try to do things to "make up for it"

Also why did you stop seeing him? If it was over a fight then allow for them to vent and try not to judge. Be sympathetic and listen. It's a great conversation starter.

And I may have missed what you are asking, do you mean how legally do you go about making the other parent allow you to see the child?