Three days ago, the Cardinals were celebrating a thrilling victory in Game 3, a triumph that put them only two wins away from capturing the 12th World Series championship in franchise history.

The town was abuzz, expectant. The sea of red was roiling. Leading the World Series two games to one, the home team had two more games at Busch Stadium and a clear opportunity to put the Boston Red Sox away. The trophy was within reach. It seemed as if nothing could stop the Cardinals now.

Except for the Cardinals, of course.

The offense has gone missing again. It’s gone into hibernation, or hiding. Welcome to another postseason mystery. Remember, the Cards scored one run, total, in losing three consecutive games (and the NL championship series) to San Francisco last season. This can’t be happening again.

More than that, the Cardinals will be trying to overcome the resistance of a tremendously proud Boston team that’s proving to be tougher, smarter and more relentless.

It’s a hardened Boston team that summoned the perfect response to its controversial, disheartening Game 3 loss to the Cardinals. A rollicking Boston team that determinedly grabbed two wins in the last two nights of the baseball season at Busch to haul a 3-2 series lead back to Fenway Park.

After Monday’s quiet 3-1 loss to the Red Sox in Game 5 before an increasingly restless home crowd, the Cards face elimination in Game 6 on Wednesday in Boston.

If the Cardinals demonstrate the kind of unwavering resolve that Boston displayed in winning Games 4 and 5, the Birds on the Bat will take this World Series to the limit and give themselves a chance to win Game 7. But if the Cardinals show any weakness at all, they’ll be swallowed up in Game 6 in the old ballyard at 4 Yawkey Way.

“You know, our guys have been backed up against the wall before, and this is something that isn’t foreign to them,” manager Mike Matheny said.

A short while later Matheny added, “I think it starts with a mentality that it’s a great challenge. It’s a great opportunity for us to go in and prove the kind of team we are as far as how tough we are mentally, and I think that’s where it begins.”

Over the past 78 seasons of major-league baseball, only four teams have gone on the road down 3-2 and won the final two games on hostile soil to seize the World Series title. The last time it happened was 1979, when the Pirates upset the expectations and the Orioles.

Things were looking promising late Saturday night, when the Cardinals won that stunning Game 3. So why do they now find themselves staring at such desperate circumstances?

Well …

You don’t have much of a chance to win a World Series when your ace, Adam Wainwright, gets outpitched by Boston’s Jon Lester in their two head-to-head matchups. After being defeated in Game 5, Wainwright is 0-2 with a 4.50 ERA in this series. Lester, 2-0, was nicked for one harmless earned run in 15  1/3 innings against St. Louis.

You’ve don’t strengthen your grip on a two-games-to-one World Series lead by losing two consecutive games at Busch Stadium for the first time since the second week of August.

You aren’t supposed to get outscored 7-3 in the two home losses by the Red Sox, who couldn’t use the designated hitter in the National League ballpark. The Red Sox weren’t able to utilize one of their top regular-season hitters, Mike Napoli, and still outhit the Cardinals — .231 batting average to .161 — in the final two games at Busch. How does that happen?

You don’t have much hope of winning the World Series when you let journeyman Jonny Gomes beat you with a three-run homer in Game 4, and get taken down by a game-winning double by backup catcher David Ross in Game 5.

You aren’t improving your prospects of winning it all by falling flat after experiencing the emotional high of being handed a sudden victory on the obstruction call that ended Game 3.

You probably aren’t destined to win a World Series when you can’t score an earned run off a sore-armed, soft-tossing pitcher, Clay Buchholz — in your home ballpark — in Game 4.

You aren’t exactly primed to earn World Series rings by averaging 2.6 runs and batting .218 with a .274 onbase percentage and anemic .303 slugging percentage in the first five games.

You aren’t increasing your odds of winning by continuing to stubbornly challenge Red Sox slugger David Ortiz, who should be dressed as a wrecking ball for Halloween. Papi had an RBI double Monday to get the Red Sox off to a 1-0 start. Through five games he’s 11 for 15 (.733 batting average) with two homers, two doubles and six runs batted in.