BOSTON — As a child, Dzhokhar Tsarnaev was quiet, hardworking and “always wanted to do the right thing,” his third-grade teacher testified Wednesday to jurors who will decide whether the Boston Marathon bomber spends the rest of his life in prison or is sentenced to death.

Catheryn Charner-Laird testified on the third day of the defense case in the penalty phase of Tsarnaev’s trial as his lawyers shifted the focus away from his older brother, Tamerlan. The defense has portrayed Tamerlan Tsarnaev, who died days after the bombing, as the mastermind of the attack.

Three people were killed and more than 260 were wounded when twin pressure-cooker bombs exploded near the finish line of the marathon on April 15, 2013.

Tsarnaev, 21, was convicted of all 30 charges against him, including 17 that carry the possibility of the death penalty. The same jury must now decide his punishment.

The defense team has focused heavily on Tamerlan, arguing that he was a domineering influence on Dzhokhar and led him down the path to terrorism. Prosecutors have said the brothers were partners in the bombing, which was designed to retaliate against the U.S. for its actions in Muslim lands.

On Wednesday, Tsarnaev’s lawyers called witnesses to testify about what he was like as a child, years before he became the Boston Marathon bomber.

“He was just learning English at that time,” Charner-Laird said, referring to Tsarnaev’s recent move to the U.S. from Russia with his family.

Tsarnaev was 9 in the fall of 2002, when he was one of her students in a combination class for third- and fourth-graders at the Cambridgeport School.

“He was incredibly hardworking,” she said. “He cared a lot about his studies; he tried very hard.”

Many times, he didn’t know what to do because of the language barrier, she said. But he “always wanted to do the right thing,” she said.

Prosecutor Aloke Chakravarty cross-examined her briefly, asking if she knew Dzhokhar to be disciplined and smart. She said he was. The question appeared designed to rebut the defense claim that Dzhokhar was under Tamerlan’s influence when he participated in the bombings.

Charner-Laird was one of several teachers who described him in glowing terms.

Rebecca Norris, one of Tsarnaev’s teachers in seventh and eighth grade, called him “really bright,” “well-behaved,’ “pretty much an A student.”

“He wasn’t a rebel. Basically, if you asked him to do something, he would do it,” she said.

Norris said Tsarnaev was one of the school’s best students and soccer players.

“I thought we would get him into a really good college with a full ride, and he would be very successful,” she said.

The defense also showed the jury two photographs of a cherubic-looking Tsarnaev from about the same time. In one photo, he is sitting on a bench next to Tamerlan, who is about 16. Tamerlan has his arm around him, while Dzhokhar rests his arm on his older brother’s leg.

In the other, Dzhokhar is smiling with his mother, two sisters and the landlady who owned the apartment building in Cambridge where they lived.

Jurors also heard from two paramedics who treated the Tsarnaev brothers after a firefight with police in Watertown days after the bombings.

Paramedic Michael Sullivan said Tamerlan Tsarnaev was combative after being shot by police and run over by Dzhokhar during a chaotic getaway attempt. Sullivan said Tamerlan “would lift himself off the stretcher and yell and scream and try to resist us touching him.”

He said sometimes people in shock react that way. Tamerlan was later pronounced dead at the hospital.

Paramedic Laura Lee said Dzhokhar was conscious and alert when he was put into an ambulance after being captured hiding in a boat in Watertown. He had a gunshot wound to his jaw, a leg wound and other injuries.

When someone put a tourniquet-like bandage on his leg, “apparently it was very tight and he was mad and it got loud,” Lee said.

“He said, ‘Where’s my brother?”‘ Lee said. She said someone else in the ambulance told him, “You’ll find out soon.”