Programs enabling conversion of the file:
If your objective is to convert the file from AAC to the file AC3, you can use any of the applications listed. AAC to AC3 conversion process in every program intended for this purpose (so-called converters) is very intuitive and everyone is able to manage converting a file.
However, bear in mind that the list may also include programs to support and work with the indicated extensions AAC and AC3 - and not only converters. In such a case, you need to open the source file AAC and from the options available in the program, choose ”Save as... AC3" or "Export to AC3".