How can I make the keys randomly change after each keypress?

Hi guys, I made a Tetris game and want to make the key for rotating the blocks change randomly after each keypress. For example: first time using the "R" button in order to rotate the block but if I want to rotate the same block again, then I need to try another button...

 void Update() {
         if (State != TetrimoState.Fixed && State != TetrimoState.Preview && State != TetrimoState.Spawning) {
             if (State != TetrimoState.Landed && Input.GetAxis("Vertical") < 0)
                 StartCoroutine(FallingDown());
 
             if (Input.GetButtonDown("Horizontal"))
                 StartCoroutine(MoveHorizontal());
 
             // Set "up" as alternative button for Jump (Project => Input)
             if ((Input.GetButton("Jump")) && Time.time > NextSwap) {    
                 if (this.CanRotate) {
                     StartCoroutine(RotateTetrimo());
                     NextSwap = Time.time + SwapCooldown;
                 }
             }
 
             // Automatic falling down
             if (NextFall < 0) {
                 StartCoroutine(FallingDown());
                 NextFall = FallingCooldown;
             }
             NextFall -= FallingSpeed * Time.deltaTime;
         }
 
         if (State == TetrimoState.Preview) {
             transform.Rotate(0, 1f, 0, Space.World);
         }