third: the one that comes third when things are counted in order, or the third part left when a whole is split into three
This is the ordinary counting word for "third," the ordinal that matches the cardinal number three the way "third" matches "three" in English. Most of the time it simply marks position in a sequence: the third day, the third hour, the third time, the third son, the third angel. Its weightiest use by far is the gospel claim that Jesus rose "on the third day," a phrase repeated so often it became a fixed part of the earliest Christian confession.
The same word also does a second job. When a whole is divided into three equal portions, "the third" names one of those portions, what English calls a third or one-third. The book of Revelation uses it this way again and again in the trumpet judgments, where a third of the earth, the sea, the rivers, the sun, and even of mankind is struck.
These two uses share one idea (the number three applied to something) but answer different questions. The first answers "which one?" and points to a place in a line. The second answers "how much?" and points to a fraction of a quantity. They are close enough to live under one word yet distinct enough that a reader should know which is meant, so they are kept here as two senses of the one word.