First of all, we know that the coins has 2 side - head and tail. And the probability of tossing and get head or tail is 50/50 or 1/2.
Next,
P(getting a head in the first toss):
1/2
P(getting a head in the second toss):
1/2
P(getting a tail in the last toss):
1/2, so the probability of getting heads in the first two trials and Tails in the last trial is 1/2×1/2×1/2 or 1/8 is your final answer. Hope it help!