.NET platform, .NET environment or .NET framework which is correct? And what's the difference between platform/environment/framework?

These are semantically identical. Usage depends only on the context where it is used:

• You build code using .NET framework (equals .NET libraries)

• The code runs on the .NET platform (in the CLR)

• You need to install .NET environment (.NET framework redistributable)

None of them is the only "correct one" while all are correct for all scenarios.

All are correct. They're essentially synonyms.

Agreed, although, I'd say general practice is to refer to it as the .NET framework.

.NET covers a lot of areas that in other ecosystems are separate entities/projects. So for example, on the Java platform, you build an app with a framework like Spring or Struts. In .Net you'd use ASP.NET MVC, or say Entity Framework, but those are both inherently part of the platform. Think of platform as "not requiring an additional install".