I use 9v battery (not 2 AAs ) because I use the same battery to supply another 9v circuit in the remote control.
The using 9 of series diodes is a good idea but it take a big area in the circuit .
And I tried to use LM7803 but it consumption current continuously without turn on the remote control.
Is there another method to convert 9v to 3v with transistor?
You can get some pretty simple boost converter ICs that just need a couple of capacitors, resistors and an inductor. Checkout Linear Technologies, Maximum Semiconductor and National semiconductor websites.