Three coins are tossed.

$\therefore$ The sample space =$2^3=8$ possible outcome

$S=\{HHH,HHT,HTH,HTT,THH,THT,TTH,TTT\}$

Let A be the event "Exactly one tail"

$A=\{HTH,HHT,THH\}$

Let B be the event "Exactly two tail"

$B=\{HTT,THT,TTH\}$

Let C be the event "Exactly three tails"

$\therefore C=\{TTT\}$

$\therefore A\cap B \cap C=\phi$ but $A\cup B \cup C\neq S$

$\therefore A,B$ and C are mutually exclusive but not exhaustive.