Answer by Lipis

Here is a simple example on how can you do it.

1. Create new Javascript and rename it to PlayerAttack
2. Copy/paste the below code into it
3. Drag the PlayerAttack script to your First Person Controller
4. Press the Play button to see it action

var fireRate: float = 2;
var comboNum: float = 3;
private var fired: boolean = false;
private var timer: float = 0.0;
private var comboCount: float = 0.0; 

function Update () {        
    if (Input.GetButtonDown("Fire1")) {
        if (!fired) {
            fired = true;
            timer = 0.0;
            comboCount = 0.0;
            Debug.Log("I served a punch!");
            //Do something awesome to deliver a punch!
        } else {
            comboCount++;
            if (comboCount == comboNum) {
                Debug.Log("I did a combo!");                
                //Do something awesome for the combo!
            }
        }
    }

    if (fired) {
        timer += Time.deltaTime;
        if (timer > fireRate) {
            fired = false;
        }        
    }
}

The basic concept of this approach is that if you pressed Fire1 you are starting a timer, and as long as the timer is less than fireRate (2 seconds in this example) you can't deliver more punches. Now during this time whenever the Player is pressing the Fire1 button you are increasing the comboCount variable. If it's exactly comboNum (3 in this example), you are ready to execute your combo function.

See also documentation for Input and Time classes.