For unreal assets like blueprints, textures, 3d models or other game assets, you can use the migrate tool to migrate those to another project.
If I were you, I would create a new project, which will be the combination project of all your 3 projects. Then I’d open up each project, select the assets I want to migrate to the new project, and migrate those. Then copy paste the source code files I want to include in the new project. And then when you do that for all 3 projects, go into the new project and fix all the issues that come up, like fixing stuff in the source code most likely, such as the namespace and maybe conflicting class names. And then in the new project you make a level with the doors where each door opens the appropriate mini-game level.