Confusion around a components pins
The middle pin that is cut and the TAB are both shorted to GND. When you are placing the part on PCB create a PCB footprint for the flat surface and connect the pad to ground. This is also used to dissipate heat. The part you have is a SMT part.

 

If you are soldering wires, solder the GND wire to the flat TAB. You will find \$D^2PAK\$ package in the attach image below. Like the TO-220 package \$D^2PAK\$ is mostly used in power products.

   

If you look at page 5 and page 9 of the datasheet you will see more infomation about the package.

References:


D2PAK Outline Dimensions 
TO-220 Package