when you are tossing 3 coins there are 8 possible combinations of coin tosses possible

these are listed below (h = head, t=tail)

H H H

H H T

H T H

T H H

H T T

T H T

T T H

T T T

of these there are 4 possible combinations where no more than one head lands up

hence the probabilty of no more than one head being tossed out of 3 coins being tossed is 4/8 or 50%