The words nucleus and nucleolus both end in -us, a fairly common noun ending in English words of Latin origin. (Latin masculine nouns of the "second declension" ended in -us in the nominative singular, and -i in the nominative plural: these forms are typically used as the English singular and plural.) Both of these words are used as nouns. More specifically, many Latin diminutive endings contain -l-, and this is what is present in the ending -olus: nucleolus is a Latin diminutive of nucleus, and this corresponds to the meaning in English which refers to a smaller body contained inside the nucleus. You can't infer the exact meaning of nucleolus from its form, but it can serve as a hint.
The word nuclear ends in -ar, a somewhat common adjective ending in English words of Latin origin. (The adjective ending -ar is a variant form of the more commonly seen adjective ending -al: typically, -ar shows up only in words where L is already present in the base.) This is consistent with the fact that nuclear is mostly used as an adjective (it can also be used as a noun, but a number of English nouns have the same form as adjectives, and can be analyzed as being derived from adjectives).
The word nucleoid ends in -oid, which is often an adjective ending, and also sometimes a noun ending, typically used to form words meaning "resembling X", "related to X" (or "something that resembles X", "something that is related to X").