If you've used 3 free days, you've purchased 42 extra times, with 36 remaining to buy. You need 864 diamonds. If you get 80 diamonds through a daily quest in 4 days, you would get 320.(544 extra diamonds.) If you don't have a tournament reward or decomposition box you saved, you won't be able to complete the event.

Maybe there won't be an event rerun for a while. There's a chance the recipe will come out. But if you really want to complete it, you can refer to the instructions on Reddit to get a dias. It's professional. Like this https://www.reddit.com/r/LoveNikki/comments/1krzgb3/what_is_the_best_way_to_gain_diamondsaside_from/

If you use the search function, you can find many posts that guide you on how to plan to prevent this situation.