i honestly think it will depend on your childs personality and your parenting style. there are plenty of people you wouldnt know were middle children but you get the occasional one where you say wooah! middle child? i know several people who have SEVERE middle child syndrome and it colours their life very strongly, one tho in particular'(cough BIL cough) would have been like that no matter if they had had a 3rd child or not, cos i know MIL's parenting would not have been the type of parenting to encourage it. i know i have several traits of middle child syndrome despite being the youngest of 5, that being said the middle child, my half sister is VERY middle child!!
in my experience (vast and educated and all that :P) the 3rd child in a family of 4 kids is the trouble maker too, so i dont know, maybe i only know cliche families? all the only children i know fit that cliche as well...