(But it has been reported that, if you do not need any external USB devices, you can tie the 5V and 3V3 rails together and feed in 3V3.)
Scary! you are describing a very good way to "let the smoke out of your PI", that is to utterly destroy it!
Never connect the 3V3 and 5V pins of the GPIO header together, it will destroy your PI if you then put 5V on your PI
And no, the PI itself (the SoC) does
use 5V! Its used by its internal switching regulator that makes a variable internal core voltage. Also the 3V3 regulator doesn't like it when it gets 3V3 on its output, with nothing at its 5V input, it might get damaged that way, so don't simply put 3V3 on the 3V3 input, and nothing on the 5V input.
Just perhaps the 5V input of the SoC will work with 3V3, but its very scary stuff to connect 5V and 3V3 together, so be extremely careful when you try! its not something I would recommend an unexperienced user to try.