I think it was supposed to be a joke about how there is only a middle child if there is an odd number of children.
That's a matter of perception.
The way I see it, every child who is not either the youngest or the oldest is the/a middle child.
With 3, that's #2. With 4, that's #2 and #3. With 5, that's #2 and #3 and #4.
Though usually the context is that parents have 3 children because that's the most common version of having more than 2 children.
When I look at people who I know and their families, I can report that the middle child always appear to be self-centered, egocentric. As if they lacked love in their youth they're continuously trying to make up for the rest of their life. Oh, and they have twisted ways of justifying that behaviour.
I was just explaining the joke the guy was trying to make.
My dad was one of 8 siblings, and my mom was one of 6. So I guess I have a lot of egocentric aunts and uncles.
BTW, 1 is also odd, so I'm not sure if you're just trying to say "used to have 3 children" or literally an odd number (1, 3, 5, 7, 11, 13, 17, 23, etc)