I don't think there's a single silver bullet. A quick google suggest about 1/3 of people commute into London by private car, which - to take your 3 million figure - means that we're already shifting 2 million people by other means. It's quite impressive when you think about it.
Again, not sure about your experience in London but it is incredible. People complain about the tube yet on the central line (my line) one arrives every 2-3 minutes rush hour each holding around 900 people. Buses galore, cyclists everywhere. The only bit that doesn't work are the roads. Scrap the single occuapncy cars and it would be a dream for commerical vehicles, cyclists and pedestrians too.
My only issue is that it does appear to be at capacity so with population increase something does need to give.
The least sutainable, most selfish and greatest danger are cars and we could easily do away with them in central London.