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.'At least two heads show'

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

Let B be the event.'At least two tails show'

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

$\therefore A \cap B=\phi$

Hence A and B are mutually exclusive events.