The existence of the 3 extra virusscanners on the TRK medium is enough to have them unpacked and ready to be used upong startup of TRK.
If you run updatetrk and your virusscanners get added to your TRK medium, you can later upgrade TRK with a newer version and still keep the antiviri available.
You can also update the antivirus signature files by manually updating them directly into the archive. But I doubt you would do it like is.