Preview button

In the figure shows the preview button in the red-colored circle.
F3 has full-aperture metering to meter, means that, whatever the aperture value it might be at that time, the view in the viewfinder is given at full aperture.
Depressing the preview button gives the preview image corresponding the aperture value at that time.
In addition, the mirror-up lever is at the base of the preview button.
The mirror rises up by depressing the preview button, and at the same time pulling down the lever.

Mechanical shutter

There is the mechnical shutter built in F3 (only 1/60sec. available).
In the figure shows the mechanical shutter lever in the blue-colored circle.
Pulling down this lever releases the mechanical shutter.