Kelly held her breath for 3 minutes, or 3*60=<<3*60=180>>180 seconds.
Brittany held her breath for 20 seconds less time than Kelly did, or 180-20=<<180-20=160>>160 seconds
And Buffy held her breath for 40 seconds less time than Brittany did, or for 160-40=<<160-40=120>>120 seconds.
#### 120