Looking for Rxjava operator to merge sources into one stream

Looking for Rxjava operator to merge sources into one stream currently have this

  Disposable observable = Observable.concat(
                service.loadPopCells().toObservable(),
                service.loadChallangeData().toObservable(),
                service.loadUserCell().toObservable()
        )              
  .subscribe(data->sendtoViewmodel(data)); // called 3 times

i have 3 stream , so on subscribe is called three time, but i want it to be called once with all data

looking to get something like this

    Disposable observable = Observable.concat(
                service.loadPopCells().toObservable(),
                service.loadChallangeData().toObservable(),
                service.loadUserCell().toObservable()
        )
        // i want to achieve something like this 
        .mapallresult(data,data2,data3){ 
         private List<SimpleCell> shots = new ArrayList<>();
         shots.add(data);
         shots.add(data2);
         shots.add(data2);
         return shots;  }
         ///
         .subscribe(dataList->sendtoViewmodel(dataList); // called once