-- тут выбираю 3 таблицы
create proc GetItem
as 
Begin
  select * 
  from [dbo].[Processors] pr
  left join [dbo].[Coumputers] c on c.[Coumputer_ID] = pr.[Computer_ID]
  left join [dbo].[Videocards] v on v.[Videocard_ID] = pr.[Videocard_ID]
End

   public partial class GetNewItem_Result
    {
        public int Processor_ID { get; set; }
        public string Name { get; set; }
        public Nullable<int> Videocard_ID { get; set; }
        public Nullable<int> Computer_ID { get; set; }
        public Nullable<int> Coumputer_ID { get; set; }
        public string Name1 { get; set; }
        public Nullable<int> Videocard_ID1 { get; set; }
        public string Name2 { get; set; }
    }

   public partial class GetNewItem_Result
    {
        public Processors Processors { get; set; }
        public Videocards Videocards { get; set; }
        public Computers Computers { get; set; }
    }