I have 3 Dell PowerEdge server, each with 2 quad-core processors. I am going to bring this office out of the stone-age network (P2P, share files on a flash drive, emails through Google, etc) and set up Active Directory and Exchange 2013. Our needs are not that great at the moment - our staff consists of approximately 40 people, and our network may eventually be managed by an external company. We need only one domain for our emails (though we may serve emails for a few other partners domains as well). I was thinking of setting something up like this:

Server 1: Primary DC. Active Directory and Exchange on separate virtual machines. Server 2: Redundant of server 1. Server 3: Shared resources, storage, backups, etc.

How would you utilize 3 servers for an Active Directory / Exchange setup for a small/medium office? We do have plans to grow, so my solution must be scalable, though I'm not sure that I want to split permissions, though I'd consider it if that was something that could be changed on down the road.