From the data sheet:-
With a 15 V input, a 5.5 V output and a 3 A load, the power is
(15 - 5.5) * 3 = 28.5 W
The output section has a thermal resistance of 2.7 °C/W, so it will be 77 °C hotter than the tab. If you are running in air at 30 °C, and the output section at its limit of 150 °C, you have 120 °C difference. 77 °C is internal, leaving 43 °C in the heat sink, so you need a heat sink that is rated at 43/28.5 = 1.5 °C/W
Something like:-
FISCHER ELEKTRONIK|SK 04/100 SA|HEAT SINK, 100MM | Farnell United Kingdom
You really should consider a buck regulator, as it will be far more efficient, and you will need a far smaller heat sink, if any. What is the application?