Bob
It depends on whether you're refering to the mapping data or the PC software (both of which could have MapSource in their titles).
The PC software is a free upgrade from the Garmin site. Without looking it up, I beleive your options are either v5.4 or v6.0.3 Beta.
The mapping data is something else. MapSource City Navigator Europe v6 is the current release and you would be eligible for a free upgrade only if you first registered your SPIII with Garmin after 1 Dec 2003 - otherwise it's US$150 for the upgrade.
Greg