Adjectives are sometimes used as sentence complements, they describe either noun or pronoun.
Ex: Gina is xenodochial. (Xenodochial there is the adjective or our sentence complement which means "friendly")
Adverbs describes verbs and another adverb. It has many kinds like adverb of matter and etc.
While prepositions are used to connect also and give us specific characteristics of a thing.
For example, : The cat is under the table. Under there is used as a preposition, if you'll study it more, under the table is a prepositional phrase. Both 3 are parts of speech that contributes and gives color and meaning in our sentwnce