In Asp.Net Core 3.0 some things have changed. For camelCase do nothing that is out of the box. For PascalCase or another set style use.

services.AddMvc(setupAction=> { setupAction.EnableEndpointRouting = false; }).AddJsonOptions(jsonOptions => { jsonOptions.JsonSerializerOptions.PropertyNamingPolicy = null; }) .SetCompatibilityVersion(CompatibilityVersion.Version_3_0); 

In Startup.cs ConfigureServices section