We can use complementary counting, and see what are the odds of 0 heads to be up. Since there are 3 coins, and there is a 50% chance to get heads or tails, the odds of having 0 heads is (1/2)^3 = 1 in 8.
Then, subtracting 1/8 from the total probability (8/8), we get the chance for at least one head to be up as 7/8.
There are 8 possibilities
only ONE ( T T T ) has no 'HEADS'
so 7/ 8 have at leat ONE head