What happens after making a firm choice?

I’m about to make a firm choice on my university option - of course this will be my first choice option. There are two other universities which have also given to me offers with conditions. So when I have made that firm choice on one of them and not accepting the other two (but did not clicked on any “reject” button), does that mean I would have declined the offers from the other two universities since I have not accepted their offers by the given deadline. Also in the event that I do not meet the conditions of the university offer that I have made my firm choice, how do I go back to one of the two other universities who have also given to me their conditional offers?