Answer: 1/2
Step-by-step explanation: This is a coin. Having heads three times in a row will never change that a coin is two sided and random, meaning it will never affect the coin to always be heads. They were most likely just lucky to get heads all the time. It will always be 1/2 since the coin is inanimate and is not like selecting classmates from a team, it cannot know it's probabilities and affect it. Thus it is 1/2 probability.
This can be simplified in the equation: P(H) = 1/2, P(T) = 1/2, P(H | T) = 1/2