In the optional rules, it says you get 3 commons votes for every controlled or captured town. Does this mean, for example, that Warden of the Northern Marches gives you 5 commons (2 for the card, 3 for Berwick)?
Yes, according to the second paragraph of Optional Parliament #1;
"Parliamentary votes are printed on the appropriate Crown cards under the heading Commons for House of Commons and Lords for House of Lords. Town and city votes are not placed on the title, office and bishop cards but must be counted on the board."