Dont get me wrong, i dont mean to insult anyone by saying this, but isnt Bosnia de facto a split country, even today? The division is different, but there is definitely no such a thing as "Bosnian" (citizen of Bosnia) nationality. Serbs live in a country called Republica Srpska, Bosniaks and Croats live in Federation, while bosnia is bascially EU financed project of union of these 2 countries... When Serbs from RS come to Serbia we do call em "Bosanci" aka "Bosnians" but that calls on their geographic origin not nationality, if u ask them a vast majority will tell u that theyre Serbs - thats at least from my experience, have in mind that alot of my family is from Banja Luka, RS.
In medieval times, even with religous difference, i really doubt ppl living there would call themselves anything other than Serbs or Croats. The only reason there could be a "Bosnian" culture in game is to be politicaly correct imo - which have very little to do with EU3.