It's likely (but not guaranteed) that the couple in question will identify as polyamorous, meaning that they are able to feel nice sexual and/or emotional stuff for more than one person at a time.

So while there will probably be a physical component to the relationship, being a couple's "third" often means you'll also go on dates and spend time with them in much the same way you would with an individual. You'll increase your chances of exciting conversation but decrease your chances of successfully splitting a main course.