'Neither' means "A is not x and B is not x, either'. As A and B are both singular nouns, the verb should be in the singular form also. This makes the third sentence grammatically wrong. Your list should read like this: both of whom are both of which are neither of whom is neither of which is As you can see, 'both' (referring to two people or things together) is plural, while 'neither' (referring to two people or things individually) is singular. That said, it is common for people to treat 'neither' as a plural noun. For example, both 'neither of whom/which has arrived' and 'neither of whom/which have arrived' are acceptable to most native speakers.