Answer by taking a quote from the following article:

Ted Williams was born Theodore Samuel Williams (after former President Theodore Roosevelt and his father, Samuel Stuart Williams) in San Diego, California on 30 August 1918. At some later date he amended his birth certificate, removing his middle name, which he claimed originated from a maternal uncle (whose actual name was Daniel Venzor), who had been killed in World War I. His father was a soldier, sheriff, and photographer from New York, while his mother, May Venzor, a Mexican-American from El Paso, Texas, was an evangelist and lifelong soldier in the Salvation Army. Williams resented his mother's long hours working in the Salvation Army, and Williams and his brother cringed when she took them to the Army's street-corner revivals.

In January 1942, after World War II began, Williams was drafted into the military, being put into Class 1-A. A friend of Williams suggested that Williams see the advisor of the Governor's Selective Service Appeal Agent, since Williams was the sole support of his mother, arguing that Williams should not have been placed in Class 1-A, and said Williams should be reclassified to Class 3-A. The attorney took the case to the Appeals Board and the board rejected the case. Angry, the attorney took the case to the Presidential Board. Williams was reclassified to 3-A ten days later. Afterwards, the public reaction was extremely negative. Quaker Oats stopped sponsoring Williams, and Williams, who previously had eaten Quaker products "all the time", never "[ate] one since" the company stopped sponsoring him.  Despite the trouble with the draft board, Williams had a new salary of $30,000 in 1942. In the season, Williams won the Triple Crown, with a .356 batting average, 36 home runs, and 137 RBIs. On May 21, Williams also hit his 100th career home run. He was the third Red Sox player to hit 100 home runs with the team, following his teammates Jimmie Foxx and Joe Cronin. Despite winning the Triple Crown, Williams came in second in the MVP voting, losing to Joe Gordon of the Yankees. Williams felt that he should have gotten a "little more consideration" because of winning the Triple Crown, and he thought that "the reason I didn't get more consideration was because of the trouble I had with the draft [boards]".  Williams joined the Navy Reserve on May 22, 1942, went on active duty in 1943, and was commissioned a second lieutenant in the United States Marine Corps as a Naval Aviator on May 2, 1944. Williams also played on the baseball team in Chapel Hill, North Carolina, along with his Red Sox teammate Johnny Pesky in pre-flight training, after eight weeks in Amherst, Massachusetts, and the Civilian Pilot Training Course. While on the baseball team, Williams was sent back to Fenway Park on July 12, 1943 to play on an All-Star team managed by Babe Ruth. The newspapers reported that Babe Ruth said when finally meeting Williams, "Hiya, kid. You remind me a lot of myself. I love to hit. You're one of the most natural ballplayers I've ever seen. And if my record is broken, I hope you're the one to do it". Williams later said he was "flabbergasted" by the incident, as "after all, it was Babe Ruth". In the game, Williams hit a 425-foot home run to help give the American League All-Stars a 9-8 win.

What does the classification 3-A mean for Williams ?
the public reaction was extremely negative. Quaker Oats stopped sponsoring Williams,