using var db = new AppDbContext(contextOptions.Options);
db.Database.EnsureCreated();

var result = db.Things
    .LeftJoin(
        db.Items
            .Select(x => new
            {
                Item = x,
                Item2Total = (decimal?)db.Item2s
                    .Where(y => y.ItemId == x.Id)
                    .Sum(y => y.Amount),
            }),
        x => x.Id,
        y => y.Item.ThingId,
        (x, y) => new
        {
            ThingId = x.Id,
            ItemAmount = (decimal?)y!.Item.Amount,
            Item2Amount = y.Item2Total,
        })
    .ToList();

public class AppDbContext : DbContext
{
    public AppDbContext(DbContextOptions options) : base(options) { }

    public DbSet<Thing> Things { get; set; }
    public DbSet<Item> Items { get; set; }
    public DbSet<Item2> Item2s { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Thing>()
            .HasKey(x => x.Id);

        modelBuilder.Entity<Thing>()
            .HasMany<Item>()
            .WithOne()
            .HasForeignKey(x => x.ThingId);

        modelBuilder.Entity<Item>()
            .HasKey(x => x.Id);

        modelBuilder.Entity<Item>()
            .HasMany<Item2>()
            .WithOne()
            .HasForeignKey(x => x.ItemId);

        modelBuilder.Entity<Item2>()
            .HasKey(x => x.Id);

    }
}

public class Thing
{
    public int Id { get; set; }
}

public class Item
{
    public int Id { get; set; }
    public int ThingId { get; set; }
    public decimal Amount { get; set; }
}

public class Item2
{
    public int Id { get; set; }
    public int? ItemId { get; set; }
    public decimal Amount { get; set; }
}