Short answers, pulled from the story.
Auguste Maquet was the ghostwriter who crafted the initial plot outlines and historical research for The Three Musketeers. Alexandre Dumas took Maquet's skeletal narratives and expanded them into the final swashbuckling masterpieces.
The Three Musketeers was first serialized in the newspaper Le Siècle between March and July 1844. This period occurred while France was still under the July Monarchy, four years before the revolution that established the Second Republic.
Alexandre Dumas claimed he found the story in a historical novel by Gatien de Courtilz de Sandras titled Mémoires de Monsieur d'Artagnan published in 1700. He presented the work as a recovered manuscript to lend the story an air of historical authenticity.
The story of The Three Musketeers is set between 1625 and 1628. This timeframe covers the adventures of d'Artagnan who traveled to Paris to join the Musketeers of the Guard during a time of intense political intrigue in France.
d'Artagnan befriends three formidable musketeers named Athos, Porthos, and Aramis. These three men became known as the three inseparables after d'Artagnan helped them fight against Richelieu's guards.