LONDON — Usain Bolt got the gold medal and another world record, too, anchoring the Jamaicans to victory in 36.84 seconds today to cap off his second straight 3-for-3 Olympics.

The United States finished in 37.04 seconds to get the sliver, the old world-record time by Bolt and the Jamaicans last year. Trinidad & Tobago got the bronze medal when Canada was disqualified after finishing third.

Bolt already had won the 100 and 200 gold medals to become the first person to go back to back in both Olympic sprints.

The race was pretty much even when Bolt and Ryan Bailey received the baton for the homestretch, but Bolt pulled away to help Jamaica set the relay record for the third time since 2008.

Meanwhile, Allyson Felix won her third gold medal of the London Olympics, giving the United States a 20-meter lead after the second leg of the 4×400-meter relay and then watching teammate Sanya Richards-Ross bring home the victory.

The U.S. runners finished in 3 minutes, 16.87 seconds for the country’s fifth straight Olympic title in the event. Russia finished second in 3:20.23 and Jamaica was third in 3:20.95.

DeeDee Trotter ran the opening leg for the United States and built a lead before handing to Felix, who more than doubled the advantage by the time she passed to Francena McCorory.

Richards-Ross had a stress-free anchor leg to add this gold medal to her 400-meter gold. Felix earlier won the 200 and 4×100 relay.