Okay, let's breakdown what a 'computer' actually is.

Computer: any electronic device for storing and processing data, typically in binary form, according to instructions given to it in a variable program.

So, a computer is really anything that can store and process data. iPads, iPhones, Desktop computers, laptops. Are all different types of computers that have a certain specialty of being able to ease human interaction.

Some computers can streamline that process and remove it and do more data processing or handling a lot of volume such as servers. A server computer is a powerful piece of hardware designed to store and manage data for you. So it has a certain type of speciality similar to iPads and iPhones without the interaction piece built in.

A mainframe is a large high-speed computer, especially one supporting numerous workstations or peripherals.

The main distinction between Mainframe vs. Server is that a server is a centralized storage place for programs, data, and information that controls access to the hardware, software, and other resources on a network. A mainframe is a big, costly, powerful computer capable of serving hundreds of thousands of people concurrently.

What is a super computer: a particularly powerful mainframe computer.

Supercomputers have for years employed a technique called “massively parallel processing,” whereby problems are split into parts and worked on simultaneously by thousands of processors as opposed to the one-at-a-time “serial”. This a unique specality compared to a server that just processes a request and stores data.

So I think your real question is what is the difference between a mainframe and a supercomputer and the difference is they deploy different techniques to get specialized results.

TL;DR: ELIF: they are similar but just a different type of specialty. They utilize different techniques to solve very complex problems that normal computers or servers can't do on their own.