Cannon 5D mark III raw files would not be supported in Lightroom 3.
You need atleast version 4.1 of Lightroom to access there raw files.
Here is the reference article : Cameras supported by Camera Raw
You can either upgrade the Lightroom version to a newer version or you can install Free DBG Converter to convert the raw files into DNG and then, access them in Lightroom.
Okay, thank you. how do I upgrade the version? I don't want to pay for CC each month at this point.
You can go for standalone Lightroom 6 if you are not willing to go for the subscription.
Here is the link to purchase : Products
You need to scroll down to Adobe Photoshop Lightroom and click on Buy and choose Upgrade