Mounting position of pixhawk

I’m building a 6 rotor hexacopter. Is there any problem if the Pixhawk orange cube is 10 cm in front of its center of gravity?

That’s fine. However, you should set that offset using the parameters EKF2_IMU_POS_X, Y, and Z.

is it enough?
Should EKF 3 and EKF 1 be changed as well?

Not sure what you mean. For PX4 there are only EKF2 params, no EKF1 or EKF3. Maybe you’re using ArduPilot and not PX4? If so, then you should consult the ArduPilot docs or their forums