On the Account page layout, I developed a Visualforce page to insert in a section. I want this section to only be able to be viewed by three specific users. Is there a way to do this?
I see two ways
Create a separate page layout for those 3 users and assign it to their profile. => take into account that any user that gets that profile as well will see the visualforce page as well
in your VF page, only show stuff for users that are allowed to see it: eg those 3 users. You can define weater or not it is allowed using a hierarchical custom setting. This concept is explained here in the context of trigger execution, but is applicable is this scenario as well. http://www.absi.be/personalize-automation-salesforce.aspx