From the Super Mario Wiki
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During the events of the game, Mario, Luigi, and Paper Mario are kidnapped and taken to Bowser's Dungeon. They encounter a large door that can only be unlocked with a key they find is on the other side of a short bridge with a sign concerning it's stability. When the trio attempts to cross it, the bridge collapses and the trio are sent plummeting, but Mario and Luigi climb up with Paper Mario in the middle with his body folded. This gives Luigi the idea to create the Trio Grab, which can be activated when the player presses and presses , , and then to form into a stack. Paper Mario can then press to grab items in front of them. To return to a exit the player must press or to exit the Trio Grab.
After the Trio Grab is learned, the icon must be pressed twice or the icon can just be touched on the bottom screen to activate the Trio Grab immediately.