I have a question about the Solenoid lock: can I use it with a 12v relay module without an external power supply?
I'm using Raspberry pi 3 model B.
I'll assume that your reference to
"a 12v relay module without an external power supply" is blether, and your question is rather how to supply 12V of power to a Solenoid-driven door lock.
The answer to that question is an unqualified "it depends on how much power is required"! Shown below are two approaches:
IF your Solenoid doesn't require more than say 100 milliamps of current at 12VDC, you may be able to add a DC-DC converter to boost the 5V RPi Supply up to 12VDC as shown below.
But unless you've got a substantial 5V RPi supply, I think the external supply may be a better option - shown below:
These schematics aren't detailed enough to actually build a circuit. That's because you didn't give enough information to support these details. Suggest you now do a bit of homework, and edit your question to make it clear enough to allow someone to help with a properly detailed answer.