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Please refer to Setting up the Development Environment - MRTK3 | Microsoft Learn to set up your development environment. The official document provides detailed implementation steps.
Please note that MRTK3 is in preview and not yet covered on Q&A. If you have more questions about MRTK3 please see the options at https://github.com/microsoft/MixedRealityToolkit-Unity#engage-with-the-community
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