void Update(){
    for (i = 0; i <= 2; i++) {
        platform.SetActive (false);
        if (i >= 1 ) {
            i--;
            platform.SetActive (true);
        }
    }
}

float blinkInterval = 1.0f; // Turn on/off every second

void Start() {
    StartCoroutine("Blink");
}

IEnumerator Blink() {
    while(true) {
        platform.SetActive(true);
        yield return new WaitForSeconds(blinkInterval / 2f);
        platform.SetActive(false);
        yield return new WaitForSeconds(blinkInterval / 2f);
    }
}

public class ActiveAlternator : MonoBehaviour {

    [Tooltip("Target object to turn on/off (NOT this object or its parents!")]
    public GameObject target;

    [Tooltip("Seconds to wait between activating / deactivating the object")]
    public float interval = 1f;

    // IEnumerator makes this run as a coroutine we can suspend & resume with yield.
    IEnumerator Start() {
         if(target == gameObject)
             Debug.LogError("ActiveAlternator cannot target itself and still re-activate");


        // Infinite loop! But it's OK, because we'll tell it to take breaks.
        while(true) {

            // This passes control back to the engine so it can draw the frame,
            // and asks to resume this method on the next line after some time.
            yield return new WaitForSeconds(interval);

            // If the object was active, set it inactive, and vice versa.
            target.SetActive(!target.activeSelf);

            // Then loop to wait, and alternate again, forever.
        }

    }
}