InnerJoin.cs

                var Records = context.Customers.Join(context.TransactionExchangeMoneys
                                                    , x => x.Cust_ID
                                                    , y => y.PayBy
                                                    , (x, y) => new { x, y })
                                               .Join(context.Products
                                                    , t=> t.y.ProductID
                                                    , p => p.ProductID
                                                    , (t,p) => new {t,p})
                                               .GroupBy(r => new { r.t.x.Cust_ID,r.t.x.UserNames ,r.t.x.EmailAddress_First,r.t.x.ShopName})
                                               .Select(g => new UserAndMoneyPaid
                                                {
                                                    Username = g.Key.UserNames,
                                                    Email = g.Key.EmailAddress_First,
                                                    ShopName = g.Key.ShopName,
                                                    TotalMoneyCollect = g.Sum(a=>a.p.MoneyReturnShop),
                                                    TotalMoneyPaid = g.Where(a=>a.t.y.ReturnMoney != null).Sum(a=>a.t.y.ReturnMoney)
                                                })
                                                .OrderBy(o=>o.Username);