2N3055 is a general purpose NPN power transistor manufactured with the epitaxial base process, mounted in a hermetically sealed metal case. The device is designed for general purpose switching and amplifier applications.
2N2646 is general purpose silicon PN Unijunction Transistor that is designed for general purpose industrial applications. The device is popular as a triggering device and is not suited for driving power loads.
2N4403 is a general purpose PNP transistor used for low power switching and amplifier applications. The NPN complementary for the device is 2N4400, 2N4401.