Chris Brown -- When In Amsterdam ...

Chris Brown is in Amsterdam ... and to prove it, he took photos of himself inside one of the city's famous "Coffee Shops" with THREE joints in his mouth at the same damn time.

Brown and some of his boys documented their marathon weed smoking session on their Instagram accounts -- advising the public, "dont worry, its medicinal."

Brown is in town to perform a show at Amsterdam's Ziggo Dome on December 6 ... if he remembers to show up.