Imperfect Employment History

Hello -

I have a question about my imperfect employment history. Since college I've held three corporate jobs in industry X with great companies that most everyone would recognize. The problem is that I was fired from the first one and fired from the last one with a successful two-year stint at the one in between. After leaving the last company I reassessed my priorities and decided to pursue a new direction which will be the focus of my MBA. For the most part I am proud of the work I did at all of these companies and my dismissals had to do with two single events of being careless rather than an immoral action or a pattern of problems.

Obviously I have to list these jobs on my resume as they make up the majority of my career, but my question is whether I should address my dismissals ahead of time within the application in the hopes the admissions council will look favorably on my honesty, or not mention them as you are not explicitly asked to do so? I don't intend to misrepresent myself but I don't want to bring up negatives if unnecessary. Should I expect a background check to reveal these dismissals, and would not mentioning those dismissals be viewed as a lie? Can I talk about those jobs at all without brining up my dismissals?

Thanks so much in advance for your help!
Grateful