Windows or Mac ? i should read closer. Generally windows will assign in the order it sees for the sata connector port 0,1,2,3...etc being C,D,E,...
There are a couple ways to do this move.
1) First clone the drive to the new one.
When you open the PC to disconnect the drive, look on the label to match the serial # and unplug the sata cable. You may have to physically remove the drive to see the label. Go into properties for the new drive, and set the drive letter to E: and make sure everything works as it should. Once that is done, then unplug the new drive , plug in the old 2TB drive and set the drive letter to a different letter. Plug in the 3TB drive. Then you can use ithe 2 TB drive however you want.
2) install the new drive and move the files over. You will have to reconnect the database if using LR. There are plenty of posts here on how to do that.