Eberhard III became the heir under guardianship in 1628 during the Thirty Years' War at the age of 14 after the death of his father, Johann Frederick, 7th Duke of Württemberg.

Eberhard fled to Strasbourg, where he married in 1637, returning to Württemberg in 1638 after long negotiations with Ferdinand III of the Holy Roman Empire.

The Duchy of Württemberg was reinstated after long negotiations resulting in the Peace of Westphalia of 1648, despite or maybe because of the effects of war, poverty, hunger and the Bubonic plague all of which reduced the population from 350,000 in 1618 to 120,000 in 1648.

Eberhard III was the second son of John Frederick, 7th Duke of Württemberg and Barbara Sophie of Brandenburg.

They had fourteen children in just seventeen years, nine of whom lived to adulthood: Secondly, he married on 20 July 1656 with Countess Marie Dorothea Sofie of Oettingen-Oettingen (29 December 1639 – 29 June 1698).