I've done a backup, actually a full backup of phone and memory using Nokia PC Suite, then i've upgraded the firmware to version 3, but i have no idea how to restore just the messaging part

i also use Best Message Storer to backup the messages, but there's no option to restore.

anyone knows how?

Sadly, partial restore options are not available anymore AFAIK, especially for the same phone model. Earlier versions of PC Suite had a few more options for both backup and restore, e.g. you could choose to backup/restore only certain data like contacts/messages/calendar, etc.

do you remember which version that is?

maybe a 3rd party software able to do partial backup and restore?