Gotta' be more, much more, to make them different ethnicities.
My goodness, they even look alike!
The Bosniaks don't even make the grade ~ they eat pork! Most of 'em who've ever emigrated to the US end up as Methodist church members (which I've always found absolutely astounding).
The Serbs and Croats are pretty much of the same bloodlines, as many converted based on where they wound up living. There are many Serbs, Croats and Muslims with the same last name. My maiden name is a very common Croat last name, even though my ancestors were Serbs.
But Serbs and Croats have very different historical references so their narratives are different.
Most Croats were protected from Islam by the Austro-Hungarian Empire, of which Croatia was a part. In the 18th century, the Austrian Empress Maria Therese even brought Serbs into Croatia to guard the border, with the promises of free land and that Serbs could keep their religion and way of life, as long as they kept the Turks out of the Austrian Empire (which is how so many Serbs originally came to live in Croatia)
On the other hand, Serbs struggled under the Turkish yoke and were pretty much on their own against aggressive Islam. And as a result, Serbs got shoved all over the map of Yugoslavia, and beyond.
My own family originated in Hercegovina, but when the Turks started killing off all the Christian nobility (anyone who could lead), they made a quick exit down to Montenegro where they have been for the last 400 years.
The vast majority of Muslims in Bosnia in the 1990's were not militant Muslims, but Alija Izetbegovic and his political leadership (including Haris Siladzic who is still in power) were. The importation of mujahadin from the Middle East to Bosnia during and after the war, radicalized some of the Bosnian Muslim populace.
But most of the Bosnian Muslims who come here to the US (with some exceptions) are not crazy. They just want a new life here -- something "normal" and to fit in. I buy some Euro foods from a store owned by a Bosnian Muslim couple here in Sacramento and they are the just sweetest people.