Before you begin, you need to assume some things about the coins for this problem to be solvable. These assumptions are natural, but they are ESSENTIAL as well, so you shouldn't make them without realizing it.
#1) You have to assume the outcomes on each coin flip are independent. (Our understanding of the physics of coin flipping easily justifies this assumption.) the answer is 1/2
#2) You must assume something about the probability of heads on each of the three coins. The usual assumption is that there is a 50% chance of heads on each toss of each coin.
