You can't use a 7805 to create 3V. Resistors are not a good option unless the load is always constant - even then, though, they are not a good option. If you want to use diodes, then the selection depends on the load current - if the diode cannot handle the amount of current required, then put them in parallel. Diodes have a forward drop of around 0.6V, so three in series would get you to about 3V.
Or you could go with the LM317, using either 12V or 5V as the input voltage (5V input wastes less power)