When using a cardinal number to form a noun phrase you can always use the definite article to identify a specific instance of a number of things, in the same way as you would with any other noun phrase.
Usually, you shouldn't use an indefinite article when using cardinal numbers with objects because the objects are usually plural, and the indefinite article is not used with plural nouns. If the number is one, this is used to replace the indefinite article as they are synonyms in this context. The exception to this is when a number is used in a compound adjective, describing the make-up of the object. Here the object can still be singular, and the number is used to describe the adjective, not to count the object.
There was one monster. - No need for "a" because "one" replaces it.
There was a one-eyed monster. - Now we need an "a" because "one" is used in the compound adjective "one-eyed" - it describes the number of eyes, not the number of monsters.
There were many monsters. / There were five monsters. - No need for an "a" because "monsters" is plural in both cases.
The five monsters terrified the town. - You are identifying a specific instance of "five monsters", so you can use "the".
In your extract, the is used because it is referring to three specific fevers (which are then listed), not just any three fevers.