The Scots came to America direct from Scotland. They differ from the others in that they did not spend any time in Ulster. They came to America from different departure points, often in different migrant waves, and settled in different areas of colonial America. For example, many Scots settled in the Chesapeake area of Virginia, whilst the Scots-Irish, generally helped to open up the western frontier in places like Pennsylvania, along the Shenandoah Valley, Virginia, and the Carolinas.
Scots-Irish, Scotch-Irish and Ulster-Scots, basically they are variant names for the same people. All three terms relate to people who left Scotland, many in the seventeenth century, settled as part of various, successive waves of plantation in Ulster, the northern-most province of Ireland, stayed maybe one, two or several generations and then moved on to North America.
From the first decades of the eighteenth century, the Scots-Irish started to emigrate to the Americas in ever increasing numbers. The migrant flow became stronger as settlers from Ulster took advantage of the opportunities in the burgeoning colonies. Having moved once already, and broken the link with their ancestral home in Scotland, it was quite practical to move again, where a better future beckoned.
Although the terms, historically, have been used inter-changeably in the Americas, more commonly these immigrants are referred to as Scots-Irish and Scotch-Irish in North America. Despite the assertion that Scotch applies only to whisky, and not to the people of Scotland, many Scotch-Irish in America are fiercely proud of this title, and defend its use unfailingly, citing evidence from the period to substantiate their claim.
The term Ulster-Scots, although also used in colonial America, is more commonly applied in the British Isles to refer to the people who moved from Scotland to Ulster, and many of whom, then some time later, moved again to America.