Volume 3 covers the final months of the siege of Boston. It opens with General Washington proclaiming the commencement of the remodeled Continental army on New Year's Day 1776 and closes at the end of March as he prepares to depart for New York in the wake of the British evacuation of Boston.

Washington's correspondence and orders for this period reveal an uncompromising attitude toward reconciliation with Britain and a single-minded determination to engage the enemy forces in Boston before the end of the winter. Washington's bold proposal to attack Boston across the frozen back bay in the middle of February was rejected as too risky by a council of war, but the council did approve occupying the strategic Dorchester Heights overlooking the city and harbor. During the last weeks of February and the first days of March, Washington devoted himself to mobilizing artillery and gunpowder for a massive cannonade of Boston and assembling materials for portable fortifications to be erected on the frozen soil of Dorchester Heights. The successful execution of this operation on the night of 4 March failedto provoke General William Howe into assaulting the American lines and thereby open the way to counterattack on the city as Washington hoped it would. It did, however, compel the British to withdraw from Boston in haste a few days later, giving Washington and his army a spirit of confidence with which to embark on the New York campaign. The volume also includes a number of documents relating to Washington's private affairs in Virginia, the most important of which are eight letters from his Mount Vernon manager Lund Washington.