Um... And the rules to each games are entirely different? They all have their emphasis.
EUIII has more rules pertaining to diplomacy, wars, and vast empire management, and is lighter on other subjects.
Victoria is more focused on products, economy, trade, society management and is lighter on other subjects.
Crusader Kings is more focused on rules about your lineage, lords, vassals, who is who's brother, etc. Skims a bit on other subjects.
So basically, they are all pausable real-time strategy on world maps during their specific periods, but they really detail rules and gameplay on different aspects of empire management.