What is the difference between Arminianism, Molinism, and Calvinism? I know the basic difference between Arminianism and Calvinism in the soteriology subject, but when Molinism comes I can't grasp the core doctrines that it teaches. I'm not asking for which is better, just a concise, easy, and helpful definition of each one without too much philosophical blather.
I found an article that explains Molinism in simple, easy to understand language.  The second part of the article asks whether it is biblical, but I have left that part out since this is not what you ask. Here is a partial quote:

Molinism is named for the 16th-century Jesuit, Luis de Molina. Molinism is a system of thought that seeks to reconcile the sovereignty of God and the free will of man. The heart of Molinism is the principle that God is completely sovereign and man is also free in a libertarian sense. Molinism partly seeks to avoid so-called “theological determinism”: the view that God decrees who will be saved or damned without any meaningful impact of their own free choice. Today’s highest-profile defenders of Molinism are William Lane Craig and Alvin Plantinga.


The primary distinctive of Molinism is the affirmation that God has middle knowledge (scientia media). Molinism holds that God’s knowledge consists of three logical moments. These “moments” of knowledge are not to be thought of as chronological; rather, they are to be understood as “logical.” In other words, one moment does not come before another moment in time; instead, one moment is logically prior to the other moments. The Molinist differentiates between three different moments of knowledge which are respectively called natural knowledge, middle knowledge and free knowledge.


1. Natural Knowledge – This is God’s knowledge of all necessary and all possible truths: all things which “can be.” In this “moment” God knows every possible combination of causes and effects. He also knows all the truths of logic and all moral truths. This knowledge is independent of God’s will, a point few if any theologians would dispute.


2. Middle Knowledge – This is God’s knowledge of what a free creature would do in any given circumstance. This knowledge consists of what philosophers call counterfactuals of creaturely freedom. These are facts about what any creature with a free will would freely do in any circumstance in which it could be placed. This knowledge, like natural knowledge, is independent of God’s will.


3. Creative Command – this is the “moment” where God actually acts. Between His knowledge of all that is or could be, and all that actually comes to be, is God’s purposeful intervention and creation.


4. Free Knowledge – This is God’s knowledge of what He decided to create: all things that “actually are.” God’s free knowledge is His knowledge of the actual world as it is. This knowledge is completely dependent on God’s will.


Using middle knowledge, Molinism attempts to show that all of God’s knowledge is self-contained, but it is ordered so as to allow for the possibility of man’s free will. In other words, man is completely free, but God is also completely sovereign—He is absolutely in control of all that happens, and yet humanity’s choices are not coerced.


According to Molinism, God omnisciently knows what you would have been like had you lived in Africa instead of Australia, or had a car accident that paralyzed you at age 9. He knows how the world would have been changed had John F. Kennedy not been assassinated. More importantly, He knows who would choose to be saved and who would not, in each of those varying circumstances.


Accordingly, it is out of this (middle) knowledge that God chooses to create. God has middle knowledge of all feasible worlds, and He chooses to create the world that corresponds to His ultimate desires. Therefore, while a person is truly free, God is truly in control of who is or is not saved. Molinists differ on how God defines His underlying desires. For example, some believe God is seeking the maximum number of people to be saved. Others believe God creates in order to maximize some other divine goal.  Source: https://www.gotquestions.org/molinism.html

As to how it differs from Arminianism and Calvinism, I doubt I could do justice to that.  Perhaps you could draw from these articles:
https://www.gotquestions.org/arminianism.html
https://www.gotquestions.org/calvinism.html
https://www.gotquestions.org/Calvinism-vs-Arminianism.html
Like Nigel J, I am simply presenting one particular point of view in the hope it will help to answer your question.  I have permission to copy and paste the Got Questions article on Molinism.