If We do not know which coin is a dime, a nickel or a quarter.
We can do permutation algorithm to reveal all possibilities
By flipping 1 coin each time
So there are at most 4 flips
example :
H H T
H T T
H T H
T T H
T T T
But If the question is to make all heads, so there are at most 7 flips
H H T
H T T
H T H
T T H
T T T
T H T
T H H
H H H