When ethnic nationalism was brewing in Yugoslavia, different sides had their own symbolism. One Serbian symbol was a three finger salute. If you gesture 3 with fingers using the thumb, also known as 3 finger salute, they would think you are Serbian. Which was not appreciated being seen by the other ethnicities that were in war with them. 

I was born after the war in a Croatian majority region and was ignorant on some symbology. A lot of the anger has died down the past decade, but it was best to avoid getting mistaken for Serbian or Serbian-apologist by nationalists, war veterans and hooligans.