Three Keys is based on an actual event in California in 1994 when Proposition 187 was passed. While this legislation would only limit opportunities for those who are undocumented/unauthorized, the immigrants in the novel are all affected by the attitude toward newcomers that the bill conveys. Mentions of Prop 187 are sprinkled throughout the text. The bill symbolizes intolerance and xenophobia on the part of native-born Californians. It relates to the theme of contrasting immigrant experiences.

The author tries to help the reader understand the legislation from the standpoint of white Californians by making Mrs. Welch the representative of their viewpoint. At the beginning of the novel, she explains to her class that taxpayers are bearing the cost of educating unauthorized immigrants to the tune of 1.5 billion dollars. Her words are given further weight by the school cutbacks that affect Mia and her classmates. They are housed in a trailer instead of a classroom, and Mrs. Welch needs to pay for school supplies out of pocket.

Mentions of Prop 187 are used to reveal character as the people surrounding Mia react to the bill positively, negatively, or indifferently. As might be expected, Mr. Yao is hostile to undocumented immigrants and votes for the proposition because he is a businessman.