How to split a string using a specific delimiter in Arduino?

I think you need a split-string-into-string-array function with a custom separator character.

There are already several sources on the web and at stackoverflow (e.g. Split String into String array).

// https://stackoverflow.com/questions/9072320/split-string-into-string-array
String getValue(String data, char separator, int index)
{
  int found = 0;
  int strIndex[] = {0, -1};
  int maxIndex = data.length()-1;

  for(int i=0; i<=maxIndex && found<=index; i++){
    if(data.charAt(i)==separator || i==maxIndex){
        found++;
        strIndex[0] = strIndex[1]+1;
        strIndex[1] = (i == maxIndex) ? i+1 : i;
    }
  }

  return found>index ? data.substring(strIndex[0], strIndex[1]) : "";
}

You can use this function as follows (with ";" as separator):

String part01 = getValue(application_command,';',0);
String part02 = getValue(application_command,';',1);
String part03 = getValue(application_command,';',2);

EDIT: correct single quotes and add semicolons in the example.