What is the probability of three pennies landing all on heads?

Each penny independently has a 50% chance of landing on heads, one-in-two.

So our possibilities for one flip are:

H

T

For two flips:

HT HH

HT TT

Meaning that for two flips we're down to 1/4. 1/2 of the time the first one comes up heads, and 1/2 of those times, the second one will do. 1/2 * 1/2 = 1/4The third flip will be the same way:

HTH HTT HHT HHH

HTH TTT HTH TTH

Again, only in the case where we already have 2 heads is it possible to get another heads and still have all heads. Therefore, only half of the ones where there were already two heads will be three heads.

1/2 * 1/2 * 1/2 = 1/8 of the time. There is a 12.5% chance of all heads.