I think this middle child thing is something of the past to be honest.
It is down to parenting, your parenting basically. And if it is something that you encountered or witnessed in your childhood, then you would be more aware of it, and it will most likely never happen in your family.
I think age gap could have a part to play... If there is a too larger gap, then yes, the last one might be more of an only child.
We have three.. 8 (dd) 4 (dd) 2 (DS)
My middle one is a more stronger personality, but she was like that before our third child came along.
Our girls are close and are both at school together (yr 3 and rec) but our little boy has slotted in brilliantly and he plays nicely with them both and/or individually. I've been the same with him, as with the girls, and we are busy, out and about meeting friends and taking each other to toddler groups! I'm really enjoying him... He's a happy, contented, independent chappy who has slotted in so nicely.
However, we have decided to go for no 4 and are due next month. This is purely due to us wanting one more... As we've always a big brood of kids. But, we were very happy and contented with 3 kids.... (No 4 is a boy, so 2 of each!)
Don't focus on the odd number side of things... Think about the more day to day, practical things....having another little one to look after and that would affect you and and your lives. Setting the clock back to zero for example... (We love this part of it, as I love pregnancy and newborns.) Think about the finance and your working etc... Think about when you want to start trying and the school year.. As in, we wanted no more than two school years between our two boys, so cracked on with the trying and bish bosh.
Good luck and enjoy! Exciting times ahead,... x