When you first log in to TM3 the first thing to set up is your location(s). To do this go to the settings cogwheel which brings you to the locations dashboard under setup. There is a location already set up called 'The Support Stronghold' and it has the various other fields filled out. You can see this in the image below.
What you need to do is overwrite this information with your location details. To do this press Edit. This then opens the edit location screen which is shown in the following image. It is important to make sure the information you are entering is correct as these details will be used throughout TM3. For example, these details will pull through when using the location merge fields when setting up communication templates. Another example is the email address entered here will be the one to appear on email communications you send out via TM3.
Once you have filled out these details click Update. If you have additional locations to add then you should click on Add which you will see on the location dashboard. You can find out how to add a location via this article.