looping_strings.pde

LoopingFile loop1;
LoopingFile loop2;
LoopingFile loop3;

void setup() {
  loop1 = new LoopingFile("level1_map.txt");
  loop2 = new LoopingFile("level2_map.txt");
  loop3 = new LoopingFile("level3_map.txt");
}

void draw() {
  println("loop1: " + loop1.currentLineNum() + loop1.nextLine());
  println("loop2: " + loop2.currentLineNum() +loop2.nextLine());
  println("loop3: " + loop3.currentLineNum() +loop3.nextLine());
}

LoopingFile.pde

class LoopingFile {
  String[] lines;
  int currentLine = -1;
  
  LoopingFile(  String nameOfFile){
    lines = loadStrings(nameOfFile);
  }
  
  int currentLineNum(){
    return currentLine;
  }
  
  String nextLine(){
    currentLine++;
    if(currentLine > lines.length - 1){
      currentLine = 0;
    }
    
    return lines[currentLine];
  }
}