As a man, your third baseman is the only other man that you would a have a threesome with thus making it you, the girl and of course your third baseman. Most of the time this person would be you best friend, your bro, or someone you are close to and have known for a while. Your third baseman is someone who wouldn't judge and someone who you know that won't make it weird or awkward afterwards.
Girl: the only way I'm having a threesome is if its with two guys,
Guy: alright Im in
Girl: better get your third baseman