im using the l298n module, and 3x 18650 batterys (near 12v total), and supplying 12v to the 12v pin of the l298 module. Can i use the 5v pin of the module to power the 5v pin of arduino? Or i need a second power supply just for the arduino? Any tips?
The L298N is a chip, and the link you provided in the comments is to the chip, not to a full H-bridge module with voltage regulator and other components. We don't have any way to answer your question about your particular set-up without more info.
If the module you have uses a voltage regulator to provide clean 5V to its solid state chips, but does not use it to drive the motors, and it provides enough extra current to drive your Arduino then you might be able to get away with using it to feed your Arduino. If the H-bridge module uses the output of it's 5V regulator to drive the motors then I would not recommend using it to power your Arduino, since as the motor is switched on and off it will cause the voltage from the regulator to bobble. At the very least you would need a reservoir capacitor across the 5V rails near the Arduino to smooth out the 5V supply.