Dr Cheng plays the three words for the Doctor and Clara while they're sitting in his office. As he explains, the founder of 3W, Dr. Skarosa, ran white noise from television signals through a “translation matrix” to hear the three words:
“Don’t cremate me”
The premise of 3W is that the dead remain conscious after their apparent death, and the Institute will invite the rich and famous to entrust them with their body after death. Keeping it within the tanks seems like a tranquil, peaceful afterlife – as opposed to people who would have to feel cremation or dissection after they died.
From Missy’s perspective, the 3W Institutes (they’re described as being all over the world) really served as seeding points: a place where she could create her initial minions, before sending them out to cover the rest of the world.