Using 6DOF_scripting to disable correlation between pitch and throttle

First time posting here,
i want to make a UAV where throttle is controlled only by joystick or certain mode - Althold etc.
Using this lua script:
Motors_6DoF:add_motor:(0,0,0,1,0,0,0,false,1)
Motors_6DoF:add_motor:(1,0,0,-1,0,0,0,false,2)
Motors_6DoF:add_motor:(2,0,0,0,1,1,0,false,3)
assert(Motors_6DoF:init(3), ‘fail’)
motors:set_frame_string(“6Dof”)

i cant seem to disable pitch from effecting servo 3.
tried ATC_ANGLE_BOOST = 0 with no success.

the main purpose is to have a UAV with 3 servos:
2 for yaw control
1 for throttle and forward

any help?