Errol Spence Jr. demolishes late replacement Phil Lo Greco in Round 3

Errol Spence Jr., right, stopped Phil Lo Greco in three dominating rounds. Steve Marcus/Getty Images

LAS VEGAS -- There are many who view welterweight Errol Spence Jr. as the No. 1 prospect in boxing, and the 2012 U.S. Olympian from Dallas did nothing to dissuade that notion on Saturday night.

Spence, a skillful southpaw with speed and power, demolished Phil Lo Greco -- a late replacement for Roberto Garcia, who dropped out Wednesday -- in a third-round knockout victory at the MGM Grand Garden Arena on the undercard of the 144-pound fight between former welterweight titleholders Adrien Broner and Shawn Porter.

Lo Greco tried to make it a physical fight from the outset by going right at Spence (17-0, 14 KOs), and he won the first round on all three judges' scorecards.

But Spence's speed and skill advantages were obvious. He began to land left hands to the head and body in the second round, and rocked Lo Greco with an uppercut and left hand. By the end of the second round, it seemed a matter of when, not if, Spence would knock out Lo Greco.

That happened in the third round. Spence dropped Lo Greco with a short right hand to the temple, but Lo Greco got up quickly. But Spence immediately rocked him with a left hand as the fight became target practice.

The 25-year-old Spence forced Lo Greco to the ropes and was teeing off on him. When he rocked him with another left hook, referee Robert Byrd stepped in and waved off the bout at 1 minute, 50 seconds.

"He was a tough fighter. I got hit with a few head-butts, which kind of threw off my game in the first round," Spence said. "I came out strong in the second and third rounds and got him out of there."

For the fight, Spence landed 73 of 142 punches (51 percent), according to CompuBox punch statistics. Lo Greco landed just 19 of 132 (14 percent).

"The short notice didn't really affect me," Lo Greco said. "Fighters fight no matter what. Nothing you can do but fight. There wasn't really one big shot. It was more the barrage of punches that ended it. I'm fine, though, and I'll come back stronger."

Spence was strong with his body attack, which is something he spends a lot of time working on.

"That was the game plan for my first opponent [Garcia] -- go to the body," Spence said. "That's always my game plan -- go to the body and the head will fall. It was working great. He was getting tired and that opened up head shots. I went upstairs and I hurt him."

Spence has talked about stepping up in competition to fight a contender, and that fight might not be far away. Pound-for-pound king Floyd Mayweather, whose Mayweather Promotions co-promoted the card, suggested this week that welterweight titleholder Keith Thurman should defend against Spence, although Thurman has a fight scheduled for July 11 (ESPN) against former titlist Luis Collazo.

"Hopefully 147-pounders took notice and hopefully in September I can fight Floyd ... err, Keith Thurman," Spence said with a laugh when he caught his slip.

Lo Greco's appearance in the fight came as a big surprise. A few days ago he was working as a sparring partner in the Las Vegas training camp of junior welterweight titlist Jessie Vargas, who is getting ready for his significant fight next Saturday, when he will move up to welterweight and take on former two-division titleholder Timothy Bradley Jr.

Lo Greco had no specific fight scheduled, and when Garcia -- mired in personal problems and having an issue making weight -- dropped out of the fight with Spence on Wednesday, co-promoters Tom Brown of TGB Promotions and Mayweather Promotions suddenly had to find a new opponent for Spence on short notice -- one who would not embarrass anyone on national network television.

Lo Greco (26-2, 14 KOs), a four-time Canadian amateur national champion and 2004 Italian Olympian (who did not get to compete in the Games because of an injury), got the call and accepted the fight (and a $150,000 purse, same as Spence) on the spot.

He was in shape, nearby, already licensed in Nevada, and close to weight, and Nevada State Athletic Commission executive director Bob Bennett approved him despite trepidation about his inactivity.

Lo Greco, 30, an Italian from Toronto, was fighting for only the second time in 22 months, having won an eight-round decision against a journeyman opponent in March. Before that, Lo Greco had not fought since suffering his first defeat in May 2013, when he got knocked down and lost a shutout decision to Porter, who went on to win and lose a world title.

Gausha easily outpoints Grajeda

Junior middleweight prospect Terrell Gausha, a 2012 U.S. Olympian from Cleveland, had an easy time with Luis Grajeda of Mexico, winning a unanimous eight-round decision.

The judges had it 79-72, 79-72 and 78-73.

Gausha, getting airtime on NBC in prime time because the Spence-Lo Greco fight ended early, dominated. He scored a heavy knockdown on a right hand in the third round. Although Grajeda was in trouble, he made it out of the round.

Grajeda (18-5-2, 14 KOs), 28, fought back hard but was outclassed by the 27-year-old Guasha (15-0, 8 KOs).

• Las Vegas super middleweight Kevin Newman (3-0-1, 1 KO) rolled to a shutout decision against Bobby McIntyre (0-2) of Tacoma, Washington. Newman won 40-36 on all three scorecards, although McIntyre gave a big effort and landed some solid shots.

• Junior welterweight Sanjarbek Rakhmmanov (1-0, 1 KO), a former amateur standout from Uzbekistan, made his professional debut in smashing style, taking out Cleveland's Brett Simmons (2-2-2, 1 KO) with a left hook to the body in the first round. Simmons took a knee and the full count from referee Vic Drakulich at 1 minute, 21 seconds