Its main facility was a transcendentally dimensional space hidden within St Paul's Cathedral, (TV: Death in Heaven) which also contained the Nethersphere, a matrix data slice within which Missy had been collecting the minds of the recently deceased. (TV: Dark Water)
3W was ostensibly founded by Dr Skarosa, who believed that white noise was telepathic communication from the dead. The name "3W" referred to three words, "don't cremate me", which Skarosa heard when he ran the noise through a translation matrix. Skarosa therefore concluded that the dead continued to feel what was happening to their bodies. This discovery lead 3W to create mausoleums all over the world, which catered to the super-rich by caring for their bodies after death.
However, 3W was in fact a front allowing Missy to convert its deceased clients into Cybermen, which were concealed from public view by being placed in dark water. When the time came, Missy released the Cybermen from the global 3W mausoleums, (TV: Dark Water) whereupon they flew into the atmosphere and disseminated Cyber-pollen over all the major population centres of Earth. (TV: Death in Heaven)