task.js

var input = [[1, 3], [9, 10], [13, 15], [4,14], [21,17], [2,7]];
//output = [1,9,13], [1,9,14], [1,9,15]

//console.log(input[0][0])
var allElements = [];//new Array(input.length);

var y = 0;
var z = 0;
for (var x = 0; x < input.length; x++){
  allElements.push(input[x][0]);
  allElements.push(input[x][1]);
  
}

var result = [];
var result2 = [];

for (var x = 0; x < allElements.length; x++){
    result[x] = [];
    result[x].push(allElements[x]);
  
}

var z = 0;
for(var x = 0; x < result.length; x++){

  for(var y = 0; y < allElements.length; y++,z++){
    
    if(x == y && (x%2 === 0)){
      y +=2;  
      if (!result2[z]) result2[z] = []; 
      if(allElements[y]&&result[x][0]){
        
        //console.log(y);
        result2[z].push(result[x][0]);
        result2[z].push(allElements[y]);
      }else{
        z--;
      }
    }else if(x == y){
      --z;
    }else{
      if (!result2[z]) result2[z] = []; 
      //console.log(y);
      if(allElements[y]&&result[x][0]){
        result2[z].push(result[x][0]);
        if(allElements[y])
        result2[z].push(allElements[y]);
      }else{
        z--;
      }
    }
  } 
} 

var result3 = [];
var z = 0;
for(var x = 0; x < result2.length; x++){
  for(var y = 0; y < allElements.length; y++,z++){
    if(x == y && (x%2 === 0)){
      y +=2;
      if(result2[x][0] && result2[x][1] && allElements[y]){
        if (!result3[z]) result3[z] = []; 
        result3[z].push(result2[x][0]);
        result3[z].push(result2[x][1]);
        if(allElements[y])
          result3[z].push(allElements[y]); 
      }else{
        z--;
      }
    }else if(x==y ){
      --z;
    }else{
      if(result2[x][0] && result2[x][1] && allElements[y]){
        if (!result3[z]) result3[z] = []; 
        result3[z].push(result2[x][0]);
        result3[z].push(result2[x][1]);
        if(allElements[y])
          result3[z].push(allElements[y]);
      }else{
        z--;
      } 
    }
  }
}


console.log(result3);