Do I have to have 3 in mainlaind europe as well as within the British Isles?
You don't need any cities in Europe.
I know from the previous response that I didn't need to found another 3 in central or southern america.
It's not more true. You need 3 cities in North and 3 cities in South America.
http://wikirhye.wikidot.com/unique-historical-victory#toc14