Pattern p = Pattern.compile("，");
Matcher m = p.matcher("aaa，sss，bbb，ccc，aaa");
int c=0,index=-1;
while(m.find()){
  c++;
  index=m.start();
  if(c==3){System.out.println(index);return;}
}

------解决方案--------------------
Java code

public class Test {

    /**
     * @param args
     */
    public static void main(String[] args) {
        String s = "asaa,ssdds,bbb,cffcc,aaa";
        System.out.println(getIndex(s, ",", 4));
    }

    public static int getIndex(String str, String c, int times) {
        int index = 0;
        String[] arr = str.split(c);
        int length = arr.length > times ? times : arr.length;
        for (int i = 0; i < length; i++) {
            index += arr[i].length();
        }
        return index + times - 1;
    }
}

------解决方案--------------------
Java code


public static void main(String[] args) {
                //方法1、正则匹配
        int num = 0;
        String s = "aaa，sss，bbb，ccc，aaa";
        Pattern p = Pattern.compile("，");
        Matcher m = p.matcher(s);
        
        while(m.find()){
            num++;
            if(3 == num){
                System.out.println(m.start());
                
            }
        }
        
        //方法2，用String的indexOf方法，每找到一个“，”，就将字符串截取，
                  //直到找到第三个，将每次下标累加即可。
        num = 0;
        String s1 = "aaa，sss，bbb，ccc，aaa";
        int location = 0;
        
        int index = s1.indexOf("，");
        while(-1 != s1.indexOf("，")){
            location += index;
            
            num++;
            if(3 == num){
                System.out.println(location);
                break;
            }else{
                s1 = s1.substring(index,s1.length());
                index = s1.indexOf("，",1);
            }
        }
    }