How to move an airplane in the Z axis?

I'm using a modified plane-flying script from the Asset Store on the player's plane. Now, it's a fine script, BUT it's hard to change the script without breaking something.

Specifically, I'm trying to give the player the ability to slide on the X and Y axis, in addition to your normal thrust on the Z-axis. But the script moves the player along the Z-axis by setting the rigidbody's velocity, and I'm trying to move the player along the X and Y axes by using AddRelativeForce. "velocity =" and AddRelative Force don't play well with each other.

How do I make the plane move properly on all three axes?

Here's the relevant code: relevant variables:

 public float speed = 20.0f; //"Base Speed", "Primary flight speed, without afterburners or brakes"
     public float speedX = 20.0f; //"Horizontal Slide Speed", "Slide speed along the X-axis"
     public float speedY = 20.0f; //"Vertical Slide Speed", "Slide speed along the Y-axis"
     public float afterburner_speed = 40f; //Afterburner Speed", "Speed when the button for positive thrust is being held down"
     public float slow_speed = 4f; //"Brake Speed", "Speed when the button for negative thrust is being held down"
 
 
 float currentMag = 0f; //Current speed/magnitude (this is the player ship's thrust magnitude, set earlier in the script)
     
 -----------------------------------------------------------------
 
 thrust coding
 
         //Apply all these values to the rigidbody on the container.
         GetComponent<Rigidbody>().AddRelativeTorque(
             (pitch * turnspeed * Time.deltaTime),
             (yaw * turnspeed * Time.deltaTime),
             (roll * turnspeed *  (rollSpeedModifier / 2) * Time.deltaTime));
 
 
 
         GetComponent<Rigidbody>().velocity = transform.forward * currentMag; //Apply speed
 
 
         GetComponent<Rigidbody>().AddRelativeForce(
             (Input.GetAxis("Horizontal") * speedX * 2),
             (Input.GetAxis("Vertical") * speedY),
             0
             )            ;

I just get the feeling that I'm missing something here, that there's some way to fix the problem by using AddRelativeForce on all axes. I KNOW there is; I achieved all-axes flying once before, before stupidly deleting my old work and having to start from scratch.

So, can anyone help a newb make planes fly?

Thanks!