William was the oldest son of the Count of Geneva, Amadeus II, and Agnès de Chalon, daughter of Jean I of Chalon from the House of Ivrea. He was born in the Region of Savoy-Maurienne, he had two other brothers Hugues, a layman, and Amédée who will become bishop of Toul from 1320 to 1330. His sister Jeanne married Guichard VI of Albon, nicknamed LeGrand, lord of Beaujeuet, his other sister Marie, married Jean II de Chalon-Arlay son of John I of Chalon-Arlay. In 1291, his father signed a peace treaty with the Counts of Savoy, to strengthen the family status by an alliance between the two families, William was betrothed into marrying a daughter of the Counts of Savoy. Thus William must marry Agnès, the daughter of Amédée V. The contract is signed at the castle Saint-Georges-d'Espéranche of the counts of Savoy, on August 31, 1297. By this agreement, the Count of Savoy gives 10,000 gold-pounds in dowry and the castle of La Corbière to load tribute and on condition that the forces of the Count of Geneva prevent any attack, and that the family of Geneva brings 4,000 gold-pounds and the bridge in front of the castle to his son, with the castles of Rumilly in Abanais, Hauteville, Alby, Charousse as guarantee as well as other pledges.
Pierre, called "Bastard of Geneva", he married Catherine of Ternier, daughter of the lord of Ternier; originating the younger branch of Geneva-Lullin and Geneva-Boringe.
Reign period
William III accession to the throne, his father Count Amadeus, establishes his will at the castle of La Balme, September 24, 1306. In this act, he designates him as his successor and specifies that these brothers, Amédée and Hugues, will receive the castles of Varey, Mornex,, Rumilly-sous-Cornillon, and Cornillon, for the vicedominus des Bornes, for the rights on the market of La Roche, and for the lands and rents which he possesses in Vaud, all under the condition that they will be able to alienate these castles and rights only in favor of the heirs of Count. His father dies on May 22, 1308, near the Vuache castle. The actual entry in function of Count William III is not known, Pierre Duparc gives the hypothesis of Jules Vuy who considered that the new count became sometime before the death of Amadeus. This observation is based on the analysis of a charter, not original, of 1308. The historian Matthieu de la Corbière; however, indicates "by a transaction concluded on May 29, his eldest son Guillaume took over". The succession raises certain concerns, especially between the young count and his mother, Agnes of Chalon. In 1306, during the writing of the will, William was a minor and therefore a regency by his mother had been anticipated, two years later, the young count is of age to direct. A transaction is set up between the two parties, on May 29, 1308, by the executor, Jean I of Chalon-Arlay, brother of Agnes, and under the auspices of the Bishop of Geneva. Three years later, a new transaction is established in favor of the count by Jean de Chalon and reducing the share of inheritance of his mother, forcing him to renounce all claims on the county. In exchange, the count undertakes to defend the possessions of his mother. This act is guaranteed in February in particular by the Dauphin John II of Viennois, the son of the count Savoy Edward and Guichard VI de Beaujeu. The reign of William III most likely was influenced by his wife from the house of Savoy, the new count commits to the path of peace with the Count of Savoy. At the peace treaty between the Count of Savoy and the Great Dauphine Béatrice of August 1308, William "vouches for his execution". On October 23, 1308, he signs with Count Amadeus V, a treaty of perpetual peace at the castle of Saint-Georges-d'Espéranche. The treaty is an opportunity for William to recognize that he holds "in fief of the Count of Savoy the castles and jurisdictions of, Alby, Hauteville, and La Corbière, as well as all that the lords of Grésy, Cessens, and Arnaud de Grandmont hold of him in Geneva ». He also ratifies past agreements, including the peace of Annemasse from 1287 or the arbitral award of 1293 passed with William, bishop of Lausanne, and Aimon of Quart. Finally, on January 21, 1312, the sons of the count of Savoy, Édouard and Aymon, sign an agreement committing the various parties to divide the barony of Faucigny, in case of death without heir of Lord Hugues. An event in April 1312 commits the signatories of the agreement to intervene. A vassal of Baron de Faucigny, Guillaume-Albi Lucinge, commits a murder, the lords of Savoy and Geneva intervene by taking the castle Lucinges and destroy. The count, resumed his old alliance, with the Dauphin of Viennois, the bishops of Geneva and Lausanne. He also pays tribute to the Dauphin of Viennois, recognizing holding in fief his county and various castles of this lord, June 13, 1316. Death and succession, on April 11, 1319, Count William establishes his will at the castle of Annecy naming heir his son Amédée. In the event that his son disappears, he makes his brothers, Hugues and Amédée his successors. In his will, he leaves a rent to his brothers, his mother, Agnès of Chalon, and his wife Agnès of Savoy, his dowry and the Valley of the Keys and the castle of Charousse. The act is supplemented by donations to the Church and its institutions.
