| Ernest Frederick III | |
| Succession: | Duke of Saxe-Hildburghausen |
| Reign: | 1745–1780 |
| Predecessor: | Ernest Frederick II |
| Successor: | Frederick |
| Regent: | The Dowager Duchess |
| Spouse: | Louise of Denmark Christiane Sophie Charlotte of Brandenburg-Bayreuth Ernestine of Saxe-Weimar |
| Issue: | Princess Juliane Princess Marie Ernestine, Hereditary Princess of Saxe-Coburg-Saalfeld Princess Christine Frederick, Duke of Saxe-Hildburghausen |
| Full Name: | Ernst Frederick Karl |
| House: | Saxe-Hildburghausen |
| Father: | Ernst Frederick II, Duke of Saxe-Hildburghausen |
| Mother: | Caroline of Erbach-Fürstenau |
| Birth Date: | 10 June 1727 |
| Birth Place: | Königsberg, Bavaria |
| Death Place: | Seidingstadt |
Ernest Frederick III Karl, Duke of Saxe-Hildburghausen (10 June 1727 in Königsberg in Bayern – 23 September 1780 in Seidingstadt), was a duke of Saxe-Hildburghausen.
Ernest Frederick was born on 10 June 1727. He was the eldest son of Ernst Frederick II, Duke of Saxe-Hildburghausen and Caroline of Erbach-Fürstenau.
Ernest Frederick succeeded his father as Duke of Saxe-Hildburghausen when he was only eighteen years old in 1745; as a result his mother, the Dowager Duchess Caroline, acted as a regent on his behalf until he reached adulthood, in 1748.
Ernst Frederick was considered to be intelligent, talented, and one of the most handsome princes of his time. He donated a library to the city, but finally his excessive prodigality in exaggerated court pomp and military splendor drew the attention of the highest places to the financial situation of his country.
The Emperor Joseph II created a debit commission under management of the Duchess Charlotte Amalie of Saxe-Meiningen and prince Joseph of Saxe-Hildburghausen, the granduncle of the duke, to investigate the demands of the creditors and adjust the incomes and expenditures to 1769. The financial situation of the country was so disastrous that 35 years duration of this commission could not repair conditions completely.
After Ernest Frederick made use in 1757 of the Münzregal (Imperial coinage regale), he was entangled in a complaint of the realm treasury. Finally, the huge fire of the city of Hildburghausen in 1779, forced Ernst Fredercik to move to his hunting residence in Seidingstadt, where he died a year later.
In the Hirschholm Palace, north of Copenhagen on 1 October 1749, Ernst Frederick was first married to Princess Louise of Denmark, daughter of the King Christian VI. They had one daughter:
In the Christiansborg Palace, Copenhagen on 20 January 1757, five months after the death of his first wife, Ernst Frederick was married for the second time to Christiane Sophie Charlotte of Brandenburg-Bayreuth. They had one daughter:
In Bayreuth on 1 July 1758, nine months after the death of his second wife, Ernst Frederick was married for the third time to Ernestine, a daughter of Duke Ernst August I of Saxe-Weimar. They had three children: