The Arizona Diamondbacks, Milwaukee Brewers and Texas Rangers all clinched play-off berths on Friday night, only the second time in Major League Baseball history that three teams won their division on the same day.
With five days left in the regular season all six divisions are locked up.
Only the two wild-card spots remain undecided.
The Diamondbacks completed baseball's most dramatic turnaround this season, going from 65-97 last year to 91-66 (with five to play) and first place in the National League West for the first time since 2007.
They did it in Kirk Gibson's first full season as manager.
"I don't care if it's your first year or your 10th year, it's what you set out to do," said Gibson, who as a player helped the Detroit Tigers and Los Angeles Dodgers win World Series championships.
The Diamondbacks earned their spot when the rookie Paul Goldschmidt lined a two-run triple in the eighth inning to beat the San Francisco Giants 3-1.
Milwaukee and Texas both had to stick around for the results of other games before they could celebrate.
The Brewers beat the Florida Marlins 4-1 at home, then waited about a half-hour until Chicago Cubs defeated the second-place St Louis Cardinals 5-1, allowing Milwaukee to secure the NL Central. It was the Brewers' first title since they won the American League East in 1982.
Ryan Braun and Cecil Fielder homered in the Brewers' win.
Fielder, who has said this probably will be his last season in Milwaukee before leaving as a free agent, took a victory lap after the Cubs eliminated the Cardinals. He hugged several of the nearly 5,000 fans who stayed to celebrate.
"I'm just glad we're able to do it," he said. "We've got a little ways to go, but I'm going to enjoy this. Hopefully, we go all the way."
The scene was similar in Texas. The Rangers beat the Seattle Mariners 5-3, and clinched their second straight AL West title about two hours later when the Los Angeles Angels lost to the Oakland Athletics 3-1.
Beaten by San Francisco in the World Series last year, the Rangers are ready to try again.
"We knew what we had to do because we've been through it before," said Ron Washington, the manager. "Now we're back and get another opportunity."
In the American League wild-card chase, the pacesetting Boston Red Sox tightened their grip on the final AL play-off spot despite being rained out against the Yankees, as the Tampa Bay Rays were beaten 5-1 by the Toronto Blue Jays to fall 2.5 games off the lead.
The Angels, who must now focus on the wild card now that Texas has clinched the AL West division, stand 3.5 games back of the Red Sox.
Terry Francona, the Boston manager, said he is not thinking about whether his job will be in jeopardy if the Red Sox fail to grab the wild card.
"The organisation not only has the right, but it's their obligation to get the right person, the person they think is the best if at some point they think it needs to be somebody else," Francona told reporters.
"Other than that, I think it's disrespectful for me to spend one waking moment to think about my situation. We need to win games."
Theo Epstein, Boston's general manager, said only that the play-offs are still within the Red Sox's reach.
"We still have an opportunity to get where we want to go," Epstein told reporters. "It's all on us to turn this thing around. It's time to step up and show what we're made of."
In the National League, the Atlanta Braves beat the Washington Nationals 7-4 to extend their lead in the NL wild-card race to three games over the Cardinals and five over the Giants.