The unifying factor between the three philosophers is that they all affirm ethics as the pursuit of the highest good. They, however, hold divergent views on the definition of the highest good. There are clear similarities between the philosophers, but these are shared between pairs.
The theories of Aristotle and Kant both affirm the dominant role of the agent/individual over the results of their actions. Thus, the character of the agent and their reasoning determine their moral values.
Aristotle and Kant also agree that there are actions that are intrinsically condemnable. Such actions must automatically be avoided and rejected by the agent.
Kant and Mill concur that morality is the agent’s duty. Thus, the agent is responsible for performing good deeds as defined by the two philosophers.
The theories of Aristotle and Mill are similar because they based their ideas on the agent’s quest for happiness.
Aristotle: "We always choose happiness as an end in itself and never for the sake of something else.” (C.U.A)
Mill: "Actions are right in proportion as they tend to promote happiness; wrong as they tend to produce the reverse of happiness.” (C.U.A)