PRTG Traffic Grapher version 6 can not import historic monitoring data from version 3. You must first upgrade to V4.
In order to upgrade your sensor settings and historic monitoring data from PRTG Traffic Grapher V3.x to PRTG Traffic Grapher V6.x you must first upgrade to version 4 because version 6 can't import V3 databases.
You can use the Freeware Edition of PRTG 4 to load and convert your data. During this time monitoring will only be available for one sensor. After the upgrade to version 4 you can then upgrade to version 6 and your data will again be converted, now into a V6 database.
Please remember that in order to monitor all your sensors you must purchase an upgrade to V6.
Here is the V4 freeware download: download.paessler.com/download/prtg4.zip
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.