3.3V, 5V and 9V pins on Arduino
The link you provide looks rather old and I don't think it applies to UNO Revision 3 as it has more pins than on the diagram you show.

I suggest you rather take a look at this Arduino UNO link which contains uptodate information about the UNO; in there you will see that for R3, the "power rail" has been extended and has the following pins, left to right:


1 unlabeled pin (can't remember what it is)
1 IOREF pin (used by shields to know if they should operate as 3.3V or 5V devices)
3.3V regulated output
5V regulated output
2 GND pins
Vin pin: this pin can be used in 2 different ways:

either as a voltage supply input (instead of using the power plug or the USB); the input voltage should be in 7-12V range and will be regulated internally (by the board circuits) to 5V
or an output supply voltage (if you plugged voltage supply through the power plug or the USB plug) that is a "copy" of the voltage input through the power plug or USB plug (not regulated yet)