Learn Italian: Avete tre moto costose? - Do you have three expensive motorcycles?

Moto is an abbreviation of the feminine noun motocicletta. In Italian, clipped or abbreviated words are invariable, meaning their endings do not change in the plural. Therefore, one motorcycle is una moto and three are tre moto.

If moto ends in -o, why does the adjective costose end in -e?

Even though it ends in -o, moto is a feminine noun because its full form (motocicletta) is feminine. Since there are three of them, we need the feminine plural form of the adjective costoso, which is costose. The adjective always agrees with the true gender and number of the noun, regardless of the noun's abbreviated ending.

How do we know "Do you have" refers to more than one person here?

The verb avete is the voi (plural "you") form of avere (to have). If the speaker were asking just one person, they would use the singular form, which is hai.

Where is the word for "Do" in the Italian sentence?

Italian does not use helper words like "do" or "does" to form questions. You simply use the normal statement structure and add a question mark at the end, raising your voice at the end of the sentence when speaking.