What is a suffix used to compare three things?

There isn't a specific suffix used to compare three things in English. The standard way to compare three or more things is using the superlative form of an adjective or adverb.

Here's how it works:

* Adjective: The superlative form is usually created by adding -est to the adjective (e.g., biggest, smallest, fastest).

* Adverb: The superlative form is usually created by adding -est to the adverb (e.g., quickest, slowest, furthest).

Example:

* Comparative: This book is *bigger* than that one.

* Superlative: This book is the *biggest* of all three.

Note: Some adjectives and adverbs have irregular superlative forms (e.g., good - best, bad - worst).