Now the headroom is a huge problem, that I didn't think of. At the risk of sounding now completely ridiculous - is it possible to move the entire main deck flooring up a tad?
The most successful entrepreneurs do the same business that existing companies do, only better...
I am sceptical that anything along the lines you suggest would be profitable, and am sceptical that it would not have been considered and discarded by others already.
Cargo holds are extremely valuable for cargo - do not underestimate the revenues earned from that space as it is not unused.
To convert a 747 to three decks, you would face uneconomic costs of deck raising (it is not a void), air conditioning movements, building in emergency egress, and finally sourcing an aircraft.
There is a reason that old aircraft are uneconomic to fly further (heavy maintenance check costs become uneconomically high).
Finally, if your idea is that good, it would be unreplicatable. As James Caan has frequently said: if an idea can be copied easily, it is just not that good...