First of all, you can not learn everything about flutter in 3 days but you can learn most of the things needed to make a simple app.
I am a data scientist and wanted to implement google PoseNet model on a mobile application. I have heard about flutter before and I always wanted to learn it so I combine both objectives.
One more thing, I already knew other programming languages like python, C, C++, and Java. That helps a lot during the process of learning flutter.
I have used Ubuntu operating system for development so the installation process may not be for other operating systems. I would suggest you visit the official page of flutter for more information about installation and set up
I have followed some YouTube playlist and flutter documentation to get familiar with dart and flutter. In my experience dart is very easy to learn if you already know other programming languages.
Once you get familiar with basic things you should start working on projects. I learned most while doing the project. During the project I faced very specific problems and solved them with the help of doc and stack overflow. To be honest you can't learn and master everything about a language so best way to Learn while doing.
If you already don't have project idea search on GitHub and start with a project you like.
In the beginning you can try to replicate a project from GitHub then slowly modify it according to your need.
Most importantly have fun while coding so don't think too much about usefulness of the apps in the beginning.