I am trying to create a robotic arm. The problem is that I could not run all 6 servo motors -3 MG966r servo motors and 3 MG90S servo motors - simultaneously, using 12V and 3A AC to DC adaptor. Last time I tried, My arduino uno is totally broken. If you can, Could you provide the schematics for the circuit? or Is there any methods to solve this problem?. Also Is it a better idea to use servo motor shield?
For this task you should use an extra power supply not running trough the Arduino. For this kind of setup I use PCA9865 16-channel servo dirver board cost around 5$, needs two pins only and can run all servos (strong 5V power supply provided => Amperage) simultaniously. For a max of 8 servos a 5V/2-2.5A power supply (wall wart) should be sufficient.
See this example circuit for how to connect:
The PCA9865 library has examples how to drive servos via program. As this board is an I/O board you could also attach joystick(s) or buttons to it to save pins on your board.