LM2991 Negative Low Dropout Adjustable Regulator
The thermal TAB is internally connected to INPUT at device pin 3 through the package lead-frame. No external connection between the TAB and device pin 3 is required, but having one is OK.
Additionally, either the TAB, or device pin 3, or both, can be used as the INPUT connection.
However, for LM2991 (and most other negative voltage regulators) do NOT connect the thermal TAB to GROUND as this will short INPUT supply to GROUND through the internal lead-frame connection.