We started today by making a list of all of the characters in the novel. As we looked at them on the board we imagined many different ways we could group them together - by gender, by type of relationship, by connection with Meursault, by profession, etc. This is the jumping off point for your essay - to select three characters, explore the ways in which they are similar to / different from Meursault, and to reach conclusions about him based on your analysis.
You can think of this essay as reaching a conclusion deductively - juxtaposing Meursault with the three characters you select and then interpreting the results to reach conclusions. You can also start with an idea (choosing all of the female characters, or the characters named by their job / ethnicity instead of a name, or the characters involved in relationships with others) and then see where your analysis leads.