The first rule is always that listening in on the conversation of others is rude, even if you are friends with both parties. Interrupting a conversation to interject yourself in on it is also rude, unless you have a good reason to do so. Where the “don’t be afraid to be a third wheel” comes in is, it’s okay to have a private conversation with one of the parties of the conversation on your own, and ask about weekend plans, say you were also thinking about doing blah-blah and suggesting you do it together. I personally find Carolyn Hax too flippant to be taken seriously