You can continue to target .NET Framework 4.7. You only need include .NET Standard in your project. See this passage below from our article, Migrate your application to use the Azure Cosmos DB .NET SDK v3
The SDK targets .NET Standard 2.0 that unifies the existing Azure Cosmos DB .NET Framework and .NET Core SDKs into a single .NET SDK. You can use the .NET SDK in any platform that implements .NET Standard 2.0, including your .NET Framework 4.6.1+ and .NET Core 2.0+ applications.
I can't say why you are experiencing access errors. I would check the modifiers on your classes and variables, more details here