多个silverlight异步调用和组合框源

我有3个组合框和每个组合框需要不同的来源。对于每个源，我都有WCF服务，并且每个服务都在同一时间异步调用。我的问题是，当最后一个服务完成时，每个组合框都是填充的，它们都在等待最长的服务。当他的服务完成时，我需要填充combobox。

public ArtiklViewModel Artikl;
public ItemNewEdit(ArtiklViewModel _artikl, bool _edit)
{
        InitializeComponent();
        Artikl = _artikl;
        SifarnikServiceClient service1 = new SifarnikServiceClient();
        service1.GetSifarniksByVrstaAsync(1);
        service1.GetSifarniksByVrstaCompleted += new EventHandler<GetSifarniksByVrstaCompletedEventArgs>(service_GetSifarniks1Completed);
        SifarnikServiceClient service2 = new SifarnikServiceClient();
        service2.GetSifarniksByVrstaAsync(2);
        service2.GetSifarniksByVrstaCompleted += new EventHandler<GetSifarniksByVrstaCompletedEventArgs>(service_GetSifarniks2Completed);
        SifarnikServiceClient service3 = new SifarnikServiceClient();
        service3.GetSifarniksByVrstaAsync(3);
        service3.GetSifarniksByVrstaCompleted += new EventHandler<GetSifarniksByVrstaCompletedEventArgs>(service_GetSifarniks3Completed);
}
    void service_GetSifarniks1Completed(object sender, GetSifarniksByVrstaCompletedEventArgs e)
    {
        combo1.ItemsSource = e.Result.ToList();
        if (Artikl != null)
            combo1.SelectedValue = Artikl.JmjID;
    }
    void service_GetSifarniks2Completed(object sender, GetSifarniksByVrstaCompletedEventArgs e)
    {
        combo2.ItemsSource = e.Result.ToList();
        if (Artikl != null)
            combo2.SelectedValue = Artikl.GrupaID;
    }
    void service_GetSifarniks3Completed(object sender, GetSifarniksByVrstaCompletedEventArgs e)
    {
        combo3.ItemsSource = e.Result.ToList();
        if (Artikl != null)
            combo3.SelectedValue = Artikl.TarifaID;
    }