Context.Stocks
.GroupJoin(Context.Suppliers, st => st.SupplierId, s => s.Id, (st, s) => new {st, s})
.SelectMany(st=> st.s.DefaultIfEmpty(), (st, s) => new {st.st, s})
.GroupJoin(Context.Products, sts => sts.st.ProductId, p => p.Id, (sts, p)=> new {sts, p})
.SelectMany(ext => ext.p.DefaultIfEmpty(), (ext, p) => new
                   {ProductCode = ext.sts.st.ProductCode,
                   SupplierName = ext.sts.s.Name,
             ProductDescription = p.Description
                               ...})

(st, s) => new {st.st, s})