Delta robot programming help

i recently created a delta robot with 3 rc servos and controlled with the arduino duemilanove. i was easilly able to create pre-programmed movements and get it to do simple repetitive tasks. but i would like to controll it with a joystic type controller(shown i pitcures) it has a 2 axis joystick and a single linear pot.

i would like to use the joystick to control the x and y axis(foreward-backward,left-right) and use the linear pot to control up and down. the problem is is that the robot has all three axis on the same plane. so i need help trying to convert thes 3 seperate axis to controlling 3 planar axis. any help would be apreciated. thanks

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