bro! zoosiek is right. you are a bit confused in this.
Start of Goal 42 (whether you have unlocked Mythic girl or not by last fight = your goal count is 0 or 1) - > after 8hrs you fight Mythic monster again and get 5 Mythic cards for it) -> repeat it two times more to finish this goal at 24hr (by fighting Event Mythic Monster 3 times after you have unlocked Mythic girl).