Creating formula for combining several columns

I need help creating a formula. I have:

Column A Husband's First Name (John)
Column B Wife's First Name (Mary)
Column C Last Name (Smith)

So, Column D needs to be:

John & Mary Smith ~ Which I can do with no problem.

However, if John doesn't have a wife, how do I make Column D John Smith instead of John & Smith?

Oh my goodness!! Thank you, thank you!! It works!!