What are these SMD components with 3 leads?
The package is called "SOT-23" (small outline transistor), and it is used for all kinds of small-signal semiconductor devices, including BJTs, MOSFETs and diodes of all types (single and dual).

Since they are very tiny, only a small amount of information can be printed on them for identification. These tend to be manufacturer-specific strings of letters and digits, and tracing them back to a specific device is somewhat of a black art. There are websites that can help with this.

As Andy says, the reference designator on the board can offer some general clues about where to look.


D generally denotes a diode (or pair of diodes)
ZD would be a zener diode specifically
Q generally denotes a transistor (could be BJT or MOSFET)
M would generally be specifically a MOSFET
U might be some kind of integrated circut, such as a voltage reference or low-power regulator