The vast majority of South America was conquered and colonized by the Spanish and Portuguese, with the exceptions being Suriname, and British and French Guyana. Why were these colonies formed right next to each other? Why were these the only places where the British, French and Dutch established colonies in South America?
The area between the Orinoco River and the Amazon Delta was commonly called the "Wild Coast" by the English - reflecting its reputation as a dangerous and uninhabitable region. Although Columbus explored the area to the east of the Orinoco, the Spanish showed relatively little interest in the Wild Coast, preferring to build their colonies on the ruins of densely-populated state societies such as the Aztecs and Incas - which gave them easy access to tribute and labor.
When other European powers began establishing colonies in the Western Hemisphere in the late 16th century, they chose places on the peripheries of Spanish power - mostly in North America and the Caribbean, but also on the Wild Coast. The French established a tentative foothold at Cayenne in the early 17th century, while Walter Raleigh conducted explorations in Guyana in the 1590s - but the main players at this time were the Dutch.
Remember that the United Provinces were in the midst of an 80-year campaign for independence from Spain that had begun in the 1560s. Beginning in 1602, the Dutch extended this campaign to include a war against Portuguese overseas possessions (Portugal and Spain were then united into a single crown), including their trading stations in the Indian Ocean, Pacific - and in South America.
The result was that by the early 1630s, the Dutch had gained possession to most or Brazil (renamed New Holland), while establishing a number of settlements in Guyana, which were essentially trading posts.
The eastern part of the Wild Coast remained uncolonized, however, the English seized the opportunity by establishing a colony near present-day Paramaribo in the 1650s - which effectively became the personal fiefdom of Barbados' royalist governor, Francis Willoughby.
The Dutch and English fought a war in the 1660s, which ended with the Treaty of Breda, in which the colony of New Netherlands (New York) was ceded to the English in exchange for Surinam (formerly Willoughbyland), which was given to the Dutch.
Over the next century and a half, the French, Dutch and British engaged in warfare for control of the sugar plantations of the region (in the same way that they fought for control of islands in the Caribbean), which helps to explain the colonial patchwork that you allude to in your question.
