How do you picture yourself three years from now? is idiomatically correct. Another possibility though is, What do you picture yourself as three years from now?

This is a common mistake grammatically even among native English speakers. However, it is technically correct to use a singular pronoun, even in the case you described. Whether you want an apple or an orange, it will taste good. In your example,

I have recently been debating with someone about the true nature of passive voice. While we both understand that passive voice is when the subject of a sentence receives the action, like "he was hit by the ball", we can't agree about