In chronological order, so I don’t have to bother with order of importance:
1) Harriet Tubman—

Harriet Tubman was born into slavery in 1822; when her master descided to sell some of her slaves in order to pay her late husband’s debts, Harriet Tubman feared that she would be sold into the Deep South. In order to avoid this fate, she decided to escape, and was helped north by Quakers and the Underground Railroad. When she reached the North, she became active in the Underground Railroad, helping approximately 70 slaves escape. Despite extensive efforts to capture her, she was never captured. During the Civil War, she helped hundreds more slaves to escape. Following the Civil War, she became an activist for the rights of African-Americans and women.
2)Rosa Parks—

On December 1, 1955, after working her shift at a department store, Rosa Parks boarded a bus in Montgomery, Alabama. As the bus filled, she was ordered to give up her seat to a white man. Year later, Parks recalled "When that white driver stepped back toward us, when he waved his hand and ordered us up and out of our seats, I felt a determination cover my body like a quilt on a winter night…When he saw me still sitting, he asked if I was going to stand up, and I said, 'No, I'm not.' And he said, 'Well, if you don't stand up, I'm going to have to call the police and have you arrested.' I said, 'You may do that.'" Her arrest sparked the Montgomery Bus Boycott that marked the end of the segregated south, and the beginning of the civil rights movement.
3)Dian Fossey—

Dian Fossey was an American behavioral scientist who studied Mountain Gorillas in Rwanda. Disturbed by the poaching of the gorillas she was studying, she launched a campaign to stop the poaching. In retaliation, she was murdered py the poachers; the person who is thought to have masterminded her murder was also behind the Rwandan genocide.
I could easily have chosen more than three, but three was the number of the counting, and the number of the counting was three. Armaments Chapter One, verses nine through twenty-seven.