public class SampleViewModel : ViewModelBase
{
  private bool isBusy;
  public bool IsBusy
  {
    get { return isBusy; }
    set { isBusy = value;
            RaisePropertyChanged("IsBusy"); }

  private string prop1;
  public string Prop1
  {
    get { return prop1; }
    set { prop1 = value;
            RaisePropertyChanged("Prop1"); }
  }

  private string prop2;
  public string Prop2
  {
    get { return prop2; }
    set { prop2 = value;
            RaisePropertyChanged("Prop2"); }
  }

  private string prop3;
  public string Prop3
  {
    get { return prop3; }
    set { prop3 = value;
            RaisePropertyChanged("Prop3"); }
  }
  
  public SampleViewModel()
  {
     new Task(() => LoadProp1Async()).Start();
     new Task(() => LoadProp2Async()).Start(); 
     new Task(() => LoadProp3Async()).Start();
  }

  private void LoadProp1Async()
  {
     Thread.Sleep(5000);
     Prop1 = "a";
  }

  private void LoadProp2Async()
  {
     Thread.Sleep(15000);
     Prop2 = "b";
  }

  private void LoadProp3Async()
  {
     Thread.Sleep(10000);
     Prop3 = "c";
  }
}

<toolkit:BusyIndicator BusyText="Loading..." IsBusy="{Binding IsBusy}" />