This is the circuit diagram of 3V to 9V DC converter which capable to deliver output voltage about 10.4v on no load and 9.6v @30mA. By using this circuit, You will be able to replace a 9v battery from 3V DC input.
The advantage is the voltage stays over 9v for the life of the cells, while a normal 9v battery drops to 7v very quickly.
The output voltage is set to 9-10v by the 6k8 and 390R resistors. The 470R gives the circuit an idling current of about 20mA and the spikes are about 75mV.
By increasing the 470R, the quiescent current decreases but the voltage drops more when the current is 30mA.
I hope this 3V to 9V DC converter circuit useful for you. Good luck.