Knowledge of the situation, Knowledge of the aftermath, Knowledge of the coding

My thoughts would be,

1. Good core knowledge on the language we choose.
2. Better knowledge on life-cycle of the technology or platform we build for.
3. Finally, a very good understanding on project requirements.

Before I throw in my 2c worth, these would be the questions I ask:

Are you a web or a desktop programmer? Do you focus on front or back end? What are the tools you will be using on a daily basis. What is the product you are working on?

Once you have answered these questions, the concepts you need to know will become clear to you in no time, because you will be asking the right questions :)

A big one for me is Composition ("has a") vs Inheritance ("is a") in OOP. A car has wheels. A monkey is an animal.

My personal top three should be something like:

• Be humble. Everyone makes mistakes. Every programmer writes bad code once in a while. Swearing at the code and blaming others is unhealthy. Avoiding this kind of behavior is very hard but must be done (I'm still struggling with it).
• KISS > DRY. Very often, keeping things simple is better than avoiding repetition. Always write code for other people to read. Keep methods small and chose meaningful names. Try to write code that matches the verbal explanation you would give if someone would ask you "what does this code do?".
• Keep related code close. It's VERY painful to maintain code that is spread across multiple files, in multiple directories. I'm not saying one should mumble the code all in one place. But mind the separation of stuff in your project. Different functionalities might be on different places, but one functionality should preferably be in one place, be it a file or a directory.

1. How computers work: Program execution, memory management, interrupts, architecture etc.
2. Databases and Filing: Data, the food for programs. one have to know how to interpret and manipulate it.
3. Algorithms: This is what i think changes the way how people think. let it be then doing daily chores, polishing shoes, cooking a meal, designing a database, framing the next biggest building or a new medicine that change the world. THIS TEACHES HOW TO GET THINGS TO WORK.

With these 3 concepts clear, any programmer can change platform, language

Lets split the programmer in 3 pieces: Pro, Program, Grammer

1. Pro

Always be professional. Do focus while coding in standard way.

1. Program

Focus on single language and do program with many concepts and different tasks. And then only go with other language when you feel "I'm okay with this language".

1. Grammer

Do focus while coding like with semicolon, comma, naming conventions, etc.

You can see all 3 lists have the word focus. So the best top 3 concept for every programmer is focus, focus, and focus.

You might also be confused why did I split the programmer to pro, program, and grammer instead of pro, gram, and mer. Its because we should add sugar on our skill so first one is pro and then we add sugar gram on pro thus pro+gram is program and third we add sugar mer on gram and hence gram+mer is grammer. By which you know every programmer steps are adding sugar to previous skills.

1. Software engineering principles

Principles, principles and again principles. In every science, in every field, in every aspect of life - everything is based on principles. Know the 10 principles and not 10 000 impementations.

Simplicity, reusability, security, maintainability, design patterns, architecture patterns, principles like KISS, SOLID, DRY and many others.

Functional programming, OOP

The Internet, Web, networking, TCP/UDP, HTTP, HTTPS

This question on Quora has some good answers.

2. Support

Every software engineer should know how to debug, log, test, measure benchmarks, handle error/exceptions, write good comments, documentation, make Pull Requests and other git basics, make a transactions, optimize, fix bugs, refactor code, refactor architecture, receive notifications about system health, errors, new versions released, maintain and support the system in general.

3. Personal skills

Primarily:

1. Communication
2. Ability to ask a lot of right questions
3. Problem solving thinking
4. Ability to learn new things fast

If a problem is not completely understood, it is probably best to provide no solution at all.

English skills and ability to name variables, functions and everything else correct.

After that Social science, psychology, management, writing, patience.

Also business skills, business analysis, system analysis.

top3... thats hard esp. to put an order on them.... I will start with the classics.... ACID & SOLID which are basic principles used in software design.

For me those are the most important principles :)

The next thing is time complexity :) and i could go on :) but 3 it is :)