It's in Goldenrod City. Across the train tracks, & to the left, the building on the far left will show you the stair case to the "basement". Go down a bit, & the skinny hallway on the right will lead you to the room you need to unlock with the Basement card/key.
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All three on!
3,2,1
its behind the radio tower next to the name raters house but you need the basement key from the rocket executive
Actually its 3,,2,1 and u get it all unlocked
The amulet coin is in the Goldenrod Department Store Basement (you get to it through Team Rocket's hideout in the underground path).