Triaxial Speakers contain three speakers, usually a low-range response unit at the top, with a mid-range response unit in the middle and a high-range response speaker at the bottom. It might also be flipped around, depending on the manufacturer, while a coaxial system usually provides either a low-range speaker and a mid-range speaker in the same box or a mid-range and high-range speaker. The key difference is that triaxial systems deliver more fidelity from multiple source points than coaxial speaker systems. Triaxial speaker systems allow sound designers to use the entire vehicle as part of the sound system more effectively than coaxial speaker designs.
For more info, see 2 Way vs 3 Way Car Speakers