Help identify Mystery Modules (RF?)
These could be a lot of things, but my first guess is they are IR receivers.  That fits with the three pins.  They are power, ground, and data output.  The data outputs of these things tends to be open collector, pulled low when IR with the right modulation frequency is received.

These modules contain active circuitry that looks for modulation in the received IR over a narrow frequency range.  Individual units are set to a fixed frequency.  The available frequencies are usually in the 30 to 45 kHz range.

After a few cycles of the modulation frequency is detected, the output is pulled low.  The IR transmitter sends bursts usually about 10-30 modulation cycles long.  The data is encoded in the lengths of the bursts and the gaps between them.  This is how most TV remotes, for example, work.