The TIP120 is a NPN Darlington Power Transistor. It can switch loads upto 60V with a peak current of 8A and continuous current of 5A. This makes it suitable for medium and high power electronics like controlling motors, solenoids or high power LEDs.
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.