The JSON value could not be converted to System.Int32

For Asp.Net Core 3.0, it uses System.Text.Json for serialization and deserialization.

For using old behavior, you could use Json.NET in an ASP.NET Core 3.0 project by referencing Json.NET support.

Short Answer:

1. Install Microsoft.AspNetCore.Mvc.NewtonsoftJson which is preview version.
2. Change to services.AddControllers().AddNewtonsoftJson();