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\$ areis 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.
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