unity – How do I add a third axis of movement for my character to go up and down?

My movement system at the moment is based off of a tutorial I followed. The tutorial is about moving with spherical gravity. I want to add another axis of movement so my character can fly up and down with jet propulsion, but I cant get it to work, here is a couple parts of the tutorial code below:

void Update()
        Vector2 playerInput;
        playerInput.x = Input.GetAxis("Horizontal");
        playerInput.y = Input.GetAxis("Vertical");
        playerInput = Vector2.ClampMagnitude(playerInput, 1f);

        if (playerInputSpace)
            rightAxis = ProjectDirectionOnPlane(playerInputSpace.right, upAxis);
            forwardAxis =
                ProjectDirectionOnPlane(playerInputSpace.forward, upAxis);
            rightAxis = ProjectDirectionOnPlane(Vector3.right, upAxis);
            forwardAxis = ProjectDirectionOnPlane(Vector3.forward, upAxis);
        desiredVelocity =
            new Vector3(playerInput.x, 0f, playerInput.y) * maxSpeed;

        desiredJump |= Input.GetButtonDown("Jump");

        transform.rotation = Quaternion.LookRotation(forwardAxis, upAxis);


void AdjustVelocity()
        Vector3 xAxis = ProjectDirectionOnPlane(rightAxis, contactNormal);
        Vector3 zAxis = ProjectDirectionOnPlane(forwardAxis, contactNormal);

        float currentX = Vector3.Dot(velocity, xAxis);
        float currentZ = Vector3.Dot(velocity, zAxis);

        float acceleration = OnGround ? maxAcceleration : maxAirAcceleration;
        float maxSpeedChange = acceleration * Time.deltaTime;

        float newX =
            Mathf.MoveTowards(currentX, desiredVelocity.x, maxSpeedChange);
        float newZ =
            Mathf.MoveTowards(currentZ, desiredVelocity.z, maxSpeedChange);

        velocity += xAxis * (newX - currentX) + zAxis * (newZ - currentZ);

How can I add another axis? I already have a third axis control for the trigger buttons set up in the input project settings, but I just cant find a way to fit a rising and falling axis in here. Help would be much appreciated