Spawn number diffrent from two previous ones

In this script i am creating a random number diffrent from the previous one. I would like it to be random number diffrent from the TWO previous ones but i can't get it to work. Does someone know the answer to this?

void SpawnElement() {

    prevSpawnIndex = prefabIndex;

    while (prevSpawnIndex == prefabIndex)
    {
        prefabIndex = Random.Range(0, prefabList.Count);
    }

    moveHelper = Instantiate(prefabList[prefabIndex]);

}

Thanks in advance!