It goes by cleric level, not character level. Otherwise people would just be able to dip into every class that had one of these abilities. You could dip into druid and get a 3rd domain and be super broken.
it means as a 6th level cleric, as you do not gain domain powers if you take levels in another class. All the domains reference your cleric level when talking about what level you gain a domain power.
Does this mean level 6 cleric or just level 6
Multiclassing: "Note that there are a number of effects and prerequisites that rely on a character's level or Hit Dice. Such effects are always based on the total number of levels or Hit Dice a character possesses, not just those from one class. The exception to this is class abilities, most of which are based on the total number of class levels that a character possesses of that particular class."