Can the three heroes who have come out of Hearthstone's legendary competitive mode be changed?

You can't change it. Jjc is characterized by random heroes and each round of cards is one of three choices. The test is your understanding and tactical execution of the profession and related cards.

Therefore, if you are not familiar with any of your three choices, I suggest you try to choose a career and practice in contact mode and combat mode. In fact, other occupations are also very interesting. I started as a priest and gradually became an absolute mage and knight. Priests are not very funny now.