Answer:

The three countries that merged together to form the United Kingdom (UK) or England are England, Ireland, and Scotland.

1. England:
England is the largest and most populous country within the UK. It is located in the southern part of the island of Great Britain and shares borders with Scotland to the north and Wales to the west. England has played a significant role in the formation and development of the United Kingdom, and it is often considered the dominant country within the union.

2. Ireland:
Ireland, which includes both Northern Ireland and the Republic of Ireland, is an island located to the west of Great Britain. Historically, the entire island of Ireland was part of the United Kingdom. However, due to political and social reasons, the southern part of the island gained independence from the UK in 1922 and became the Republic of Ireland. Northern Ireland, on the other hand, chose to remain part of the UK. Therefore, currently, only Northern Ireland is part of the United Kingdom.

3. Scotland:
Scotland is located in the northern part of the island of Great Britain. It shares a border with England to the south and is known for its unique culture, history, and legal system. Scotland has its own devolved government and has been part of the United Kingdom since the 1707 Acts of Union, which united the Kingdom of Scotland with the Kingdom of England.

Formation of the United Kingdom:
The United Kingdom was formed through a series of historical events and political agreements. The Act of Union in 1707 united the Kingdom of Scotland with the Kingdom of England, creating the Kingdom of Great Britain. This merger was driven by various economic, political, and military factors.

Later, in 1801, the Kingdom of Great Britain merged with the Kingdom of Ireland, forming the United Kingdom of Great Britain and Ireland. However, in 1922, the majority of Ireland gained independence from the UK, leaving only Northern Ireland as part of the United Kingdom. As a result, the name was changed to the United Kingdom of Great Britain and Northern Ireland, which remains the official name of the country to this day.

In conclusion, the UK or England was formed by the merger of England, Ireland, and Scotland. While the Republic of Ireland is no longer part of the UK, Northern Ireland still remains within the United Kingdom.