All three of the products you mention serve different needs.
Active Directory is used to manage users and user groups. For instance, in order for someone to log onto your network they would need a network account, which the IT Admin would have to set up for them. The account can be another admin account, allowing them access to all network resources, or it can limit the user account to see and interact with files that they have permission to and nothing else. You can set up user groups as well, to aid in configuring access. For instance, you might have a "marketing" user group. Make the user accounts for the marketing staff members of the Marketing group and they all have the same access to the same information on your network.
SharePoint is a way to manage content on your network. It puts a web-like face to the internal network, allows you to develop document libraries and applications (like a time card app that people would use to log in and out, or a help desk app where people can report problems and IT staff can log their progress in fixing the issue).
Terminal Server allows users to log into a server and use its resources. You can set it up so that when a user logs in (using their AD account) it gives them access to their files and folders and applications. This can be resource intensive depending on how many people are logged in at one time. A better solution (for laptop users at least) is VPN. It allows their laptop to log into the network and they can work the same as if they were there in the office.
I would say you have to start with Active Directory. It is the foundation for your network services. Second priority would be off-site access which can be accomplished with Terminal Server or VPN. Third, use SharePoint to make navigation of your network easier and to help organize company resources. SharePoint is not a must have for your organization, though I think it is a great addition.
