LONDON -- By the time Allyson Felix was done doing her part, her third gold medal of the Olympics was all but hanging around her neck.
Staking the US team to more than a 2-second lead at the halfway point Saturday night, then watching Sanya Richards-Ross bring home the blowout victory, Felix added the 4x400-meter relay gold to those she had won earlier in the 4x100 relay and 200-meter sprint.
‘‘By the time I got the stick,’’ Richards-Ross said, ‘‘it was basically a victory lap.’’
The United States finished in 3 minutes, 16.87 seconds — good for a 3.36-second rout over Russia, the biggest margin in the final of the long relay at the Olympics since East Germany beat the US by 3.58 seconds in 1976.
Jamaica took third in 3:20.95.
The US extended its winning streak in this race to five straight, dating to 1996.
Felix became the first US woman to win three golds in Olympic track since 1988, when Florence Griffith-Joyner won the 100, 200 and 4x100 relay in Seoul.
Felix’s victories came half a world away, though she’s now in the same stratosphere with some of the US greats.
‘‘London is very special to me,’’ Felix said.
The gold Felix really wanted was in the 200-meter sprint that eluded her in Athens and Beijing. After that, everything else was gravy in Britain, though Felix was hardly going through the motions in her last race of the games.
Handed about a 10-meter lead by teammate DeeDee Trotter, Felix ran the second leg in 47.8 seconds — 1.8 seconds faster than Russia’s Antonina Krivoshapka — to put a huge swath of track between her and the Russians before she handed off to Francena McCorory.
McCorory expanded the lead by another .49 seconds, then delivered it to Richards-Ross, who was basically running alone, loosely holding onto the baton as she breezed across the finish line.
‘‘The ladies were phenomenal,’’ Richards-Ross said. ‘‘They made it too easy for me.’’
When it was over, Richards-Ross tucked the stick under her arm and started clapping. Then, one-by-one, Felix, Trotter and McCorory came over and the whole group embraced, huddling with their arms around each others’ shoulders.
‘‘It’s unbelievable,’’ Felix said. ‘‘I think about how I ended in Beijing, just feeling discouraged there. Four years later to have all this happen, to really accomplish every goal that I set out, is such a blessing.’’
It marked yet another success for a US track team that had pegged 30 as the goal to reach at the London Olympics. After Felix and her teammates were finished, the men’s 4x100 relay team and high jumper Brigetta Barrett both took silver to lift the US team to 29. The marathon closes out the track schedule Sunday, with 2004 silver medalist Meb Keflezighi in the race.
‘‘I think the pressure was on to go out and do what we are capable of doing,’’ Trotter said of the 30-medal goal. ‘‘I think we finally hit the mark this time. We hit the center of the target. We got it done.’’
The track meet could have been an even more rousing success for the United States had the men done more in the sprints they dominated for decades, until Usain Bolt came around.
But that’s not Felix’s fault.
And she'll leave London having accomplished the same things he did at these games: Three gold medals and one world record. Felix got hers (40.82 seconds) Friday night as part of the 4x100 relay team. Bolt got his in Saturday night’s men’s 4x100 (36.84).
Felix does have one loss — her fifth-place finish in the 100 meters to open the Olympics. That was the race she qualified for after finishing in a dead heat for third at Olympic trials, then earning the spot when her teammate, Jeneba Tarmoh, dropped out of a run-off for the spot.
Felix said she used the 100 for tuneup purposes. It turned out to be quite a good use of her time.
The 26-year-old has dabbled both in the 100 and 400 over her career, which made her that rare runner who could help her team in both relays. A chance to pick up more hardware, as well — and anyone who knows America’s history in the 4x400 knows there’s a very good chance the hardware can be of the golden variety. The US has won the 4x400 at every Olympics and world championship since 2007 and is undefeated at the Olympics since 1996.
‘‘That’s the Dream Team, all day,’’ said Trotter, who also took bronze in the 400 meters.
Some track touts, knowing that Jamaica’s Novlene Williams-Mills had handed Richards-Ross her only loss in the 400 this year, predicted Jamaica — or maybe Russia — might give the United States a run in this race.
Sounded good in theory.
‘‘On paper, it seemed like it was going to be a great race,’’ Richards-Ross said. ‘‘But by the time I got the stick, we had already dominated the race.’’
It’s a great moment for the 27-year-old Jamaican native, as well.
Richards-Ross has dual citizenship and her parents moved to the US when she was 12, in part because there were better training opportunities available in the States.
These days, she’s married to Jacksonville Jaguars defensive back Aaron Ross, who has two Super Bowl rings at home that will now share space with a few more gold medals. Richards-Ross has fought on and off the last five years with a hard-to-diagnose illness that causes fatigue and skin lesions. She fought with her own emotions after a disappointing bronze medal in the 400 in 2008.
No real stress this time, though, and the US women hardly looked winded as they danced and circled the track with American flags held high behind their heads.