"she already had a trousseau that might have been the envy of three girls"

Long before an engagement was announced or considered, young women of good standing established a trousseau, storing it in a chest in anticipation of the day they would marry.  In addition to the bridal gown, the trousseau might include gowns, linens and towels for the home, bedding, and kitchenware. Often the contents of the trousseau were fashioned by the bride herself, or by the women in her family. That Grace's trousseau might be the "envy of three girls" indicates the finery of her collection.