Ok, so:
From the problem, we know that there are two possibilities when we are flipping a coin. It could be head, or stamp.
So, we notice that flipping three heads is a favorable compound event, and the probability of any situation has the following formula:
P = (favorable compound event) / (All possible events)
If we have four coins, we notice that there are four heads and four stamps. So, if we flip the four coins, we would get 8 possible events.
Then, if 3 coins flipping a head is a favorable compound event and 8 are all the possible events, probability will be:
P = 3/8.