Flipping a coin is an independent event, meaning the probability of getting heads or tails does not depend on the previous flip. It's 1/2 or 0.5 chance every time.
When flipping a coin 3 times there are eight possible outcomes: H H H H H T H T H T H H H T T T H T T T H T T T In four of those instances you have at least 2 tails. Therefore, the chance is 50%.