The three nations that sided with the Colonials were France, Spain, and the Dutch. The degree of support varied.

France was the first to sign. They saw the British as their primary enemy in global politics and were still smarting from the Seven Years War. Once they saw that the fledgling United States were capable of beating the British in some battles, they joined in. They signed treaties of alliance, and declared war on Britain on February 6th 1778. They provided advisors, equipment, money, actual troops, and at times the French fleet. In the end this support proved vital. The climatic battle at Yorktown was decisive primarily because the French fleet was able to bottle the British up and prevented retreat or resupply. In the end the war didn't bring much to France, except a lot of debt. But it did discomfort the British a bit.

The second "ally" was Spain. They were more reluctant to end up at war with Britain. And they were concerned about the idea of colonial independence. So they never officially recognized the United States during the war. They did however unofficially support the colonials and provided arms and equipment to them. They also entered the war against Britain in support of the French, on June 21st, 1779. They got some nice territorial concessions at the end of the war, including regaining the Floridas which they had lost to Britain in 1763.

The Dutch were the last to join in. They had been providing loans (frequently underwritten by the French) and due to their "neutral" nature much of the French and Spanish assistance was arriving in North America on Dutch ships. Eventually Britain declared war on the Dutch in December 1780, kicking off the Fourth Anglo-Dutch War. Despite recognizing the colonial government and providing loans and aid, the Dutch never officially allied with the US. In the end they lost their war with the British and were forced to cede Nagapattinam in India to the British, and were also forced to open certain Dutch colonies to British trade.