On Christopher Columbus' third voyage he became the first European to see what?
The best way to answer this question is most likely to say that Columbus and his men became the first Europeans to see the mainland of South America. They were also the first known Europeans to see the island of Trinidad, but this is much less important in most people’s minds than the mainland of the South American continent.
On Christopher Columbus’s first two voyages, he only managed to reach various islands in the Caribbean. Of course, he believed that these were islands somewhere off the coast of Asia. Because he believed this, he continued to look for the continent during his third voyage. On the third voyage, he arrived at the island of Hispaniola but then went south to look for new lands. He reached Trinidad first, getting water for his ships there. He then moved along, encountered the outflow of the Orinoco River, and then was the first known European to see (and also to set foot on) the continent of South America.