Sets and Probability

Hello can I get some help with this question please

Let $S = \{1, 2, 3, \dots, n\}.$ Three subsets $A, B, C$  of $S$  are chosen at random.

(a) Find the probability that $A \cup B \cup C = S.$
 

 

and there was another part that I am also having trouble with: 

 

(b)Find the probablity that $A \subseteq B \subseteq C.$