How to create a library compatible with .NET Core 3.x, 2.x and .NET Framework

Let’s assume that you have a large library with a lot of functionality and you want to use it in three different web projects. However you have a problem, the target framework for these projects is not the same. One of them is older and it’s currently using .NET Framework 4.8, the second is newer and it’s using .NET Core 2.2 and the last one is the newest…