Human-computer interaction (HCI) has been identified as a rich task for the real-world study of psychology; however, the theoretical approaches to the psychology of HCI have narrowly focused on problem-solving (e.g., GOMS and CE+), memory (e.g., mental models and metaphors), and social interaction (e.g., perceived control). An attempt to create a broader theoretical framework integrates the three approaches to the psychology of HCI with a theory, IP3. This paper (1) discusses each of the three psychologies of HCI, (2) describes the integrative theory, IP3 (verbally, as well as by a graphical representation), (3) applies the theory to one representative research area—transfer of training, and (4) applies the theory to the interpretation of selected HCI design guidelines.