Triangle sandwiches can be obtained from the Sandwich lady random event. Triangle sandwiches can also be pickpocketed from workers around the Tower of Life ('Black-eye', 'Gummy', 'No fingers', and 'The Guns').
The triangle sandwiches was originally planned to be discontinued but were added back into the game through a few characters that is located around the Tower of Life just south of Ardougne.
The npc's can be pickpocketed for fast low level experience and free sandwiches.