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.

When you eat dirt for the first time and leave your family to build a shack in the woods