BiH = Mainly ethnic Bosniaks, found in Bosnia & Herzegovina (40%
, Montenegro (8%), Serbia (2%, Sandzak (45%))
Serbia = Ethnic Serbs, found in Serbia (85%), Bosnia & Herzegovina (38%), Montenegro (35%), Croatia (5%)
Croatia = Ethnic Croats, found in Croatia (90%), Bosnia & Herzegovina (15%)
Exceptions do exist. For example, Adem Ljajic is a talented 17-year-old playing for FK Partizan, and he may sign a contract with Manchester United within the next month. Ljajic is a Bosniak (Muslim) from Serbia, from the region of Sandzak, where almost half of the population is of Bosniak ethnicity. Nonetheless, Ljajic plays with a lot of pride for the country of Serbia and has nothing but praise for it. There are a few more examples of sportsmen who opted for Serbia instead of other countries, like Denis Sefik (Bosniak), Arpad Sterbik (Hungarian), Bojan Neziri (Albanian), Cszaba Silagyi (Hungarian), etc.
It has also been the case of Bosnia & Herzegovina, who have had a lot of Serb and Croat players on their squad over the years. It has been the case of Zvjezdan Misimovic, Sergej Barbarez, Sasa Papac, Vlado Gudelj, Mirko Hrgovic, Darko Maletic, Mladen Bartolovic, and many more.
As for Croatia, it has been a very rare thing to see Serbs or Bosniaks play for their team. I can only think of Dado Prso, who is of Serbian descent, and who retired just recently.
However, most of those cases are very rare. There are a lot of Bosnian-born players that have opted for Croatia (Davor Suker, Mladen Petric, Vedran Corluka, etc.) or Serbia (Mladen Krstajic, Ognjen Koroman, Zdravko Kuzmanovic, Savo Milosevic, etc.)
In an overall situation, most players will opt for their country by ethnicity if born in Bosnia & Herzegovina, and the same goes for people of other ethnic minorities born in Serbia, Montenegro, or wherever it may be.
BiH is always said to be a country for which only 40% of the population cheers for, and it will continue to be like this for as long as the country will exist as it is. There is way too much tension in BiH for people to start considering themselves as 'Bosnians'. Unlike in Lebanon, nothing is done to calm things down; it rather seems everybody tries to shake things up. Bosnia and Herzegovina is the only of all ex-Yu republics to become a mini version of the former Yugoslavia. Every other country is ethnically compact, except BiH, which is why it will most likely never find any kind of stability in the near future.