In Freudian theory, you have three components to your personality: the id, the ego, and the superego. The Enneagram types fall under these three components and how you deal with them. It just so happens that 3,7, and 8 are all assertive (id) types. The id is the "the impulsive (and unconscious) part of our psyche which responds directly and immediately to the instincts". So, basically, because 3, 7, and 8 are ASSERTIVE types, this means that the way they deal with their id is by asserting its presence (in contrast to withdrawn types, who tend to deal with their ego by withdrawing from the world, or compliant types, who deal with their superego by exercising restraint over their impulses). This means that anyone with a 378 tritype is more likely to be seen as very tough and competitive.
I hope that made sense. Any further questions, just ask.