It includes many improvements, including adding Windows Forms and WPF, adding new JSON APIs, support for ARM64 and improving performance across the board. C# 8 is also part of this release, which includes nullable, async streams, and more patterns.
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.NET Standard 2.1 includes types since .NET Core 2.1.
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.NET Core has been hardened for Docker to enable .NET applications to work predictably and efficiently in containers. The garbage collector and thread pool have been updated to work much better when a container has been configured for limited memory or CPU. .NET Core docker images are smaller, particularly the SDK image.
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Using Declarations
Are you tired of using statements that require indenting your code? No more! You can now write the following code, which attaches a using declaration to the scope of the current statement block and then disposes the object at the end of it.
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IEEE Floating-point improvements
Floating point APIs have been updated to comply with IEEE 754-2008 revision. The goal of the .NET Core floating point project is to expose all “required” operations and ensure that they are behaviorally compliant with the IEEE spec.
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.NET Platform Dependent Intrinsics
We’ve added APIs that allow access to certain performance-oriented CPU instructions, such as the SIMD or Bit Manipulation instruction sets.
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Cryptography
We added support for AES-GCM and AES-CCM ciphers, implemented via System.Security.Cryptography.AesGcm and System.Security.Cryptography.AesCcm. These algorithms are both Authenticated Encryption with Association Data (AEAD) algorithms, and the first Authenticated Encryption (AE) algorithms added to .NET Core.
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Other API Improvements
We optimized Span<T>, Memory<T> and related types that were introduced in .NET Core 2.1. Common operations such as span construction, slicing, parsing, and formatting now perform better.
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Applications now have native executables by default
.NET Core applications are now built with native executables. This is new for framework-dependent application. Until now, only self-contained applications had executables.
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Optimize your .NET Core apps with ReadyToRun images
You can improve the startup time of your .NET Core application by compiling your application assemblies as ReadyToRun (R2R) format. R2R is a form of ahead-of-time (AOT) compilation.
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.NET Core SDK Size Improvements
The .NET Core SDK is significantly smaller with .NET Core 3.0.

— https://devblogs.microsoft.com/dotnet/announcing-net-core-3-0/