I can combine more than one table but I can not select the columns.

Could you please provide a bit more details on the problem you're trying to solve?

var UserProductOrder = CustomTableItemProvider.GetItems<MyJoinUserItem>().Source(s => s.LeftJoin<MyJoinOrderItem>("ItemID", "UserID").LeftJoin<MyJoinProductItem>("ProductID", "ItemID")).ToList().Select(s => new
    {
        UserName = s.UserName,
        TotalPrice = s.TotalPrice
    });