There are only 3 mounting holes in Arduino, and not in a tripod configuration. I often mount PC boards on metal standoffs for easier handling and to keep the bottom of the board off any surface that can potentially cause damage, but Arduino is not stable with just the 3 legs.
The freeduino clique came up with a reasonable-sounding theory that in the original Arduino (serial, the one in the famous photograph), the voltage regulator mounting hole also served as the 4th mounting point...
I stick self-adhesive rubber feet on the bottom, after Lady Ada...
Why does Arduino only have 3 legs?
In honour of the "Isle of Man"
See the flag Isle of Man - Wikipedia
Should have been "www.arduino.im"