here is my unfinished race car script

var movespeed = 0;
var turn = 0; // plus for right, minus for left
var maxspeed = 16;
var minspeed = -8;


function Update () {
transform.Translate(Vector3.left * Time.deltaTime * movespeed);
transform.Rotate(Vector3.up * Time.deltaTime * turn);


if (Input.GetKey("a")){
turn - 3;
 }

if(Input.GetKey("d")){
turn + 3;
}

  if (Input.GetKey("w")){
movespeed + 0.5;
  }

   if (Input.GetKey("s")){
  movespeed - 1;
}
if (movespeed >= maxspeed){
movespeed = maxspeed;
}

if (movespeed <= minspeed){
movespeed = minspeed;
}
}

im getting the error "expressions in statements must only be executed for their side effects" at

turn - 3;
turn + 3;
movespeed + 0.5;
movespeed + 1;

does anyone know what the problem is?