ALOHA - Throughput and probabilities
I assumed that you have calculated the probability of successful transmit (which is about 1e-4 or something)

$$
P[trials =  3] = P[Fail] ^3 * P[Succ]
$$
$$
Average = E[X] = 1/P[Succ]
$$

Please revise or study (if you haven't) the geometric distribution