After two straight blowouts to begin the Eastern Conference finals and the news that inured Boston Celtics star Isaiah Thomas will miss the remainder of the series, the Cleveland Cavaliers entered Game 3 on Sunday as 16.5-point favorites — a spread you’ll rarely even see so high in the regular season.

It took all of 23:39 for the Cavs to build a lead big enough to cover that 17-point margin, and they led by as many as 21, but Thomas’ replacement in the starting lineup, Marcus Smart, scored a career-high 27 points to lead a remarkable comeback. It all culminated in Celtics teammate Avery Bradley drilling a game-winning 3-pointer with 0.1 seconds remaining in Boston’s stunning 111-108 victory in Game 3.

“Everybody had to step their game up tonight, especially with one of our brothers down,” Smart, who added seven assists, five rebounds and two steals, told TNT’s David Aldridge after the game. “Our love and support goes out to Isaiah. We wish he could be here, but we understand. We just kept fighting. Everybody did their part. … We all fought, we chipped away, and we came back for the victory.”

The Celtics entered the game having never led in the series, and the basketball world had penciled in a Cavaliers-Warriors three-match in the NBA Finals after Cleveland blew them out in Boston by 44 in Game 2. The C’s led 3-2 and 6-5 in the opening minutes, but the Cavs quickly took control and led by 21 on a Tristan Thompson put-back with 6:39 left in the third quarter. Everyone smelled the sweep.

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Except for the Celtics. Boston got seven 3-pointers and 8-of-14 shooting overall from Smart — a career 36 percent shooter — inspired play from reserve forward Jonas Jerebko in the second half and another late scoring flurry from Kelly Olynyk to take their first lead since the early going with 4:21 remaining.

The Celtics got double-digit scoring from six different players, including 20 points from Bradley, a double-double from Jae Crowder (14 points, 11 rebounds) and 25 combined points from Olynyk and Jerebko off the bench. Smart had never scored more than 26 points in a game, let alone the playoffs.

As a team, the C’s attempted 24 fewer free throws than the Cavaliers (36-12), but shot 18-of-40 from 3, outscored Cleveland in the paint, 36-24, and added 18 second-chance points on 11 offensive boards.

The two teams traded baskets down the stretch, and a J.R. Smith 3-pointer tied the game at 106 with 36 seconds on the clock. A Jerebko 22-footer early in the shot clock gave Boston a two-point lead and, just as importantly, a second possession once Kyrie Irving answered with a layup on the other end.

All of that set the stage for Bradley’s rim-rattling 3-pointer on the final possession. Still facing an uphill battle, the Celtics cut Cleveland’s series lead to 2-1 entering Tuesday’s Game 4. And they did it while holding LeBron James to one of the four worst playoff performances of his career — an 11-point outing on 4-of-13 shooting in which the four-time MVP finished scoreless for the fourth quarter.

The Celtics actually had their most competitive first quarter of the series, which is saying something, since they still trailed 35-24 when it was through. Cavs forward Kevin Love made his first four 3-point attempts and five of his first seven shots from distance to finish with 15 points in the opening frame.

Irving added three 3-pointers of his own and was fouled on a fourth attempt to score 12 points in the first quarter. He also assisted on four of Cleveland’s incredibly efficient 11 field goals on 17 shots (including 9-of-13 on 3-pointers) in the opening 12 minutes. All this despite LeBron committing more turnovers (2) than he made shots (1) in the first quarter. This was the Cavaliers on cruise control.

Love added a pair of 3’s on two more attempts in the second quarter, finishing the first half with seven triples on 10 tries — just one shy of Vince Carter’s playoff record of eight 3-pointers in a half and four short of Golden State Warriors guard Klay Thompson’s playoff record of 11 3-pointers for a game.

Smith’s 3 with 21 seconds left gave the Cavs 14 treys on 22 attempts and pushed their lead to 18, before a Smart layup before the buzzer trimmed Cleveland’s advantage to 66-50 at the break.

Brad Stevens reminded his team they’d seen bigger deficits than that against the Cavs, and even when Tristan Thompson bumped Cleveland’s lead to 21 midway through the third quarter, Boston’s coach continued telling his C’s they could make a run, so long as they played harder than their opponent.

Smart and Jerebko must’ve been listening. With Jerebko serving as an energetic instigator against Love, Deron Williams and any other Cavalier who crossed his wiry path during the second half …

… and Smart enjoying the night of his life, Boston closed the gap to five on a 26-10 run to end the third quarter. Smart’s seventh 3 tied the game at 95 with 5:44 to go in the fourth quarter, and a short Olynyk jumper two possessions later gave Boston its first lead since three minutes into the game.

Tristan Thompson’s offensive rebounding and unlikely 12-of-15 free-throw shooting kept the Cavs afloat as the Celtics made their run, but Boston made clutch play after clutch play — a 3-pointer and a jumper by Al Horford sandwiched around a pair of Smart free throws — to prove they weren’t about to play dead, even after a 44-point loss, despite losing Thomas, and regardless of whether the series still seems over.

“A lot of people doubted us out there — thought we were finished,” Horford (16 points, six assists) told CSN New England’s Abby Chin after logging his first career playoff victory against James in 15 tries. Asked what made the difference, he added, “Heart. This team’s got heart. We got beat bad, but it’s all about how you rebound, and we bounced back tonight.”

Game 4 tips at 8:30 p.m. ET on Tuesday night.

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Ben Rohrbach is a contributor for Ball Don’t Lie and Shutdown Corner on Yahoo Sports. Have a tip? Email him at rohrbach_ben@yahoo.com or follow him on Twitter! Follow @brohrbach