Uno R3, Adafruit Motor Shield v2.3, two thumb sticks, 3 12vdc motors.

In a nutshell - I'm woefully uneducated in most of this but I'm attempting to build an underwater ROV and have hit a major snag. My concept is to use two thumb sticks attached to an Uno R3 fitted with the Adafruit motor shield v2.3 to control three dc motors. Two (the horizontal thrusters) will be on the X/Y axis of a single controller while the other (vertical thruster) will be on the X axis of the other controller. I can make the motors run through the Uno/shield combination using the dcmotor test program. I can see the thumb sticks on the serial monitor using another program.

How do I combine all this to create the functionality I'm trying for? What is the coding? Are there visuals for the circuitry? Schematics? Fritzing diagrams? Am I attempting something that isn't feasible in the first place? I really feel I'm stumbling around in the dark at this point. Any help would be hugely appreciated.

If you have already achieved working motor control, and also already achieved working joystick reading, then I assume you have the circuitry for that worked out.

Combining 2 different sketches is a common task. Take your best shot at making it work. If your best effort does not get the job done, come back here. Post the code that makes the motors work. Post the code that makes the joysticks work. Post the code that is your attempt at making the 2 work together. Use code tags. Explain what you expected the sketch to do. Explain what it actually does and how that differs from what you expected.

You will find that many forum members will be willing to help you through the difficulty.