I want to know what is the difference between
much? for example, which are a correct sentence in below:
This is far better for you This is more better for you This is much better for you
Your second phrase, "This is more better for you" is incorrect, since we use "better" instead of "more good," just like how we use "healthier" instead of "more healthy," so to say "more better" is repetitive or redundant.
As for "far better" versus "much better," they are mostly the same, and interchangeable, but I would note that "much better" is acceptable to use alone, while "far better" should probably include the thing being compared. For example, "That outfit looks much better." OR "That outfit looks much better than the last one." Versus "That outfit looks far better than the last one."
Also, "far better" is a little more formal than "much better." And actually, "much better" can be used as a standalone sentence in dialogue. For example, if a person had just changed outfits, "There you go. Much better."