xXx - Watch Order: Release Order
The xXx franchise is a high-octane action series that follows extreme sports athlete Xander Cage as he's recruited into the world of international espionage. Known for combining death-defying stunts with spy thriller elements, the franchise delivers adrenaline-fueled entertainment with Vin Diesel's charismatic performance as the unconventional secret agent.
Note that Vin Diesel is absent from the second film, State of the Union, which instead focuses on a new agent played by Ice Cube, though the events remain connected to Xander Cage's storyline. Diesel returns triumphantly in Return of Xander Cage, which brings him back for another explosive mission. The chronological order of events matches the release order, making this the only reasonable way to watch the series.
Linear Timeline: Release Order = Story Order

xXx
Xander Cage is your standard adrenaline junkie with no fear and a lousy attitude. When the US Government "recruits" him to go on a mission, he's not exactly thrilled. His mission: to gather information on an organization that may just be planning the destruction of the world, led by the nihilistic Yorgi.

xXx: State of the Union
Darius Stone's criminal record and extreme sports obsession make him the perfect candidate to be the newest XXX agent. He must save the U.S. government from a deadly conspiracy led by five-star general and Secretary of Defense George Deckert.

xXx: Return of Xander Cage
Xander Cage is left for dead after an incident, though he secretly returns to action for a new, tough assignment with his handler Augustus Gibbons.
Other Film Series
Armor of God
Jackie Chan's globe-trotting adventure trilogy following treasure hunter Asian Hawk on international quests for ancient artifacts.
Batman
The complete Batman film saga spanning multiple interpretations from Burton's gothic vision through Nolan's grounded trilogy to modern detective noir.
Fast and Furious
Street racing saga that evolved into a globe-spanning action franchise centered on family and high-octane heists.
Kingsman
A stylish spy action franchise blending high-octane espionage with humor and British charm, following the secret Kingsman organization from its WWI origins to modern-day adventures.
Shaft
A multi-generational action franchise following the Shaft family of private detectives across five decades, from the original blaxploitation classic to modern action films.
Taxi
French action-comedy franchise about a taxi driver and a clumsy cop fighting crime in Marseille, plus an unconnected American remake set in New York.