To create a Room in TM3, select the Settings icon in the top right of the interface.
Next select the Setup drop-down, choosing Rooms.
You will then be present with this screen showing you all of the rooms you have currently assigned to TM3.
To add a new room, click on the blue 'Add' button.
This will bring up the screen below. You can now enter the description (name of the room), choose the location (where the room is registered) and also a colour to help you identify between rooms on the calendar.
Once you have completed this information, click Create.
Once the room has been created, you will see them within the Room Manager screen.
Now your new room will appear in the diary as shown in the image below: