If this is for a trip in 3 months and you’ve already been denied, I would do my best to try to get either a refund, reschedule your trip for a year in the future, or change to another destination country. In other words, you’re not coming to the USA this year.
There’s no appeal process for the current application (except for situations where there was a serious error, such as looking at the wrong person’s file). From what you’ve told us, your denial is within the realm of understandable decisions given your current circumstances and so applying again now will just result in another denial. In any case, you’ll have to say that you’ve already been denied when you apply. Your current denial and the rationale will be in your records and lying about it will get you a lifetime ban.
Instead, you need to materially change your circumstances for when you apply in a year or so: graduate, get a full time job, an apartment that you’re paying for yourself or your own house, a family, a travel record of going to other non-EU countries, etc.
This isn’t something that can happen in three months thus my advice that you give up hope of going to the USA this year.