Exchange 2000

One Exchange server for 10 people is a huge, unnecessary waste.
With T1 connections between your 3 locations, you can EASILY use just a single Exchange server. Easy.
In my opinion, the best way for you to set up your network is to set up a singleAD domain with 3 Sites, one per physical location with a Win2K server. So, 3 offices, 3 servers, 3 Sites. Each server will need to be a Domain Controller and in its own Site. The users who will be in each office will need their user accounts in their respective Site.
Now, with the Exchange server, users will just authenticate to its FQDN identifier (server.yourdomain.com) when they set up their Outlook connections. But with a single AD domain, then you only need to manage one domain on the Exchange server, since the users in all 3 Sites are still a part of the same AD domain.
So, set up Exchange after you set up your AD, list Exchange in AD (I believe you need to do this after Exchange is installed for it to be visible in AD, just like anew SQL Server), build the mailboxes, then instruct your users to point Outlook to the server name.
That is it.

I agree thet each location should be configured as a site and you may want to implement a DC at each location just to make authentication faster.

I think I would also setup one private information store with a store for each remote location. Thiswill make backup and recovery faster and create less downtime for the users.

Lou