Question 673571
It helps if you write down all the possible outcome
H H H 
T T T 
H H T
H T H
T H H 
T T H
T H T
H T T
There are 8 possible outcomes, which is {{{ 2*2*2 = 8 }}}
(1)
no heads. Only T T T fits, so
P( 0H ) = 1/8
---------------
(2)
Exactly 2 heads. There are 3 of these, so
P( 2H ) = 3/8 
---------------
(3)
Exactly 1 head or exactly 2 heads. When you see OR, you
add the separate probabilities
P( 1H ) = 3/8
P( 1H ) + P( 2H ) = 3/8 + 3/8 
3/8 + 3/8 = 3/4