In "Civil Disobedience" by Thoreau, "On the Eve of Historic Dandi March" by Gandhi, and "Long Walk to Freedom" by Nelson Mandela, how is the message of the power of an individual to bring about social reform conveyed?

In Thoreau's "Civil Disobedience," Gandhi's "On the Eve of Historic Dandi March," and Mandela's Long Walk to Freedom, identifying with other people is the way to bring about social reform. 
There are many connections between Thoreau's "Civil Disobedience," Gandhi's "On the Eve of Historic Dandi March," and Mandela's Long Walk to Freedom. Each emphasizes a collective notion of the good. They suggests that when we identify ourselves with something larger than our own identity, we are compelled to take action. We are driven to bring about social reform because we value the importance of helping others.    
In Thoreau's "Civil Disobedience," taking action to bring about social change is seen through a collective lens. Thoreau sees social reform as only possible when individuals band together.  The true success of civil disobedience is only visible when people take action as one to achieve social reform:

If the alternative is to keep all just men in prison, or give up war and slavery, the State will not hesitate which to choose. If a thousand men were not to pay their tax-bills this year, that would not be a violent and bloody measure, as it would be to pay them, and enable the State to commit violence and shed innocent blood. This is, in fact, the definition of a peaceable revolution, if any such is possible.

Thoreau's "peaceable revolution" of social reform happens when "all just men" take action together.  Thoreau believes that an individual's power increases when they see past their differences and work together towards a common goal.  They might be as different as "the fugitive slave, and the Mexican prisoner on parole" or "the Indian to plead the wrongs of his race."  Thoreau sees a person's power increasing when they unify with others in the cause for social reform.  
Gandhi's Dandi march was collective action in order to achieve social reform. Gandhi wanted to achieve change and marched with others in order to do so. He wanted people to be unified in their action, and to see their interests as interlinked with others.  In his speech, Gandhi argues that his task "shall be done if I perish and so do my colleagues."  He envisions social reform as individuals sacrificing themselves for something greater.  Gandhi repeatedly talks about how he, as an individual, is not as important as the cause of challenging an unjust authority through civil disobedience. Gandhi stresses how he and his volunteers are unified in this purpose.  Adding to their unity is their faith in civil disobedience.  In Gandhi's mind, the "stream of civil resisters will flow unbroken" when people take action together. Gandhi sees social reform as possible when individuals yield to a larger cause that encompasses more than just their own interests.
Finally, Mandela believes that the power of the individual to bring about social change happens when people keep an eye to maintaining the social order. Mandela believes that when people are optimistic, they identify with others in the struggle for social change:  

I am fundamentally an optimist. Whether that comes from nature or nurture, I cannot say. Part of being optimistic is keeping one's head pointed toward the sun, one's feet moving forward. There were many dark moments when my faith in humanity was sorely tested, but I would not and could not give myself up to despair. 

Mandela views excessive individualism with "despair" and "dark moments."   In his own narrative, social reform was visible when he saw his interests rest with others' happiness:  "There was no particular day on which I said, Henceforth I will devote myself to the liberation of my people; instead, I simply found myself doing so, and could not do otherwise."  Mandela sees individuals' power towards social reform increase when they see "the chains" on" one person as "the chains on me."  Collective identity is the only path through which a wide level of change can happen. His writings and life underscore the importance of seeing beyond oneself in order to create change.