"Half of six is three."
Translation:La mitad de seis es tres.
Nope, the subject is half, which is singular. Six is the object of the preposition of. In English, the object of a preposition (or of a verb) cannot be the subject of the verb. How this works for a native Spanish speaker may be different, but the ones who've written on here seem to agree that son (the plural) does not sound correct.