The free trial will search the whole drive as long as the running user has access to all files/folders.
You don't mention how you arrive at a 3TB file size? Is the file count the same?
FileLocator Pro will include file sizes for every file that it sees during the file scan. However, there are a few reasons why the totals might not match other tools. If FileLocator Pro is unable to enumerate the files/folders in a folder, e.g. the running user does not have security privileges, then it will not be able to traverse to those subfolders.
Another reason file sizes might not match is that if you are searching through ZIP or PST file types the sub-file sizes will be included in the totals as well as the parent container file sizes.