When 3 coins are tossed simultaneously, the probability of having at most 3 heads is:

$\frac{1}{8}$

The total no. of possible cases in tossing three coins = (2)³ 

                                                                                           ^{= 8}

The possibilities in tossing three coins are = {(H,H,H), (T,H,T), (T,T,H), (T,T,T),                                                                  (H,T,H), (H,H,T),(H,T,T), (T,H,H) }

There is only possibility of getting three heads is = (H,H,H) = 1

So, the required probability = 1/8