Sp3cial K don't read any further if you don't want to know the truth.
If it was an HR dept. doing the background check, I would say you may have a chance to slip in. However, as you stated it is a 3rd party doing the check, spend the rest of your weekend revising your resume to state the accurate information and start applying for other positions. The process has already started so you can't even save face by withdrawing your application.
I use 3rd party companies to check out final candidates and I can assure you they are very thorough. Even though we ask for highest level education (ie MBA) they will still provide information on other degrees.
For previous employers they don't contact just one person, they contact multiple people.
If they call you out for your lies, do not make excuses. Be humble and explain you realize you were wrong and you have since corrected your resume. Thank them for their time and consider it a life lesson.
People talk to others in the same industry. I talk to others and we share horror stories about candidates. (without names)
Good luck.