n = 3 is the number of times the coin is tossed. Since there are 2 outcomes per toss, this means that there are 2^n = 2^3 = 2*2*2 = 8 outcomes total. Therefore, there are 8 leaves total.
If we make
H = heads
T = tails
Then here are the 8 outcomes in the sample space
HHH
HHT
HTH
THH
HTT
THT
TTH
TTT