The Inseparables

The Inseparables is the name given to the four Musketeers, Athos, Porthos, and Aramis. According to Athos, he, Aramis, and Porthos were called the "Inseparables" because they were always seen together. 
Cosette later becomes part of the Inseparables, revealed in Entry 3, "The Homecoming," where she is referred as one of them when Athos and her are talking to each other.