loop to Executor Service

I want to use JavaExecutor Service for the below:

for(String s : multiMap.keySet()){
    for(String s1 : listB){
      for(String s2 : listC){
      //API call
      }
    }
 }

as in for loops the time taken for completing is too much and is taking for ever. So i am thinking of using this in an executorService so that the tasks can be completed sooner. Can any one please suggest how to do this using Executor service ?

Note : Using Java 7.