"I don't matter" does not mean "I don't care" "I don't matter" means that you mean nothing to someone. "I don't care" means you are not worried about something. "I don't mind" means that something does not bother you. "It doesn't matter" means that something is not a problem.

These expressions, in most cases, can be used interchangeably. However, to answer your question you would not say "I do not matter" when you mean to say "I do not care". "I do not matter" is like saying "No tengo valor" and "I do not care" is like saying "No me importa". There are some subtleties between "I don't mind" and the other two (which have virtually the same meaning) but you will learn those nuances with practice. They are a bit difficult to explain. Sorry, but I hope I have helped a little.

"I would not matter" and "I don't care are two different expression. When you say I don't care, you are saying that nothing matters to you. "I would not matter" means that you are not important.

I can't think of any differences between these three expressions.

*differences