First off, you are entitled to one free copy of your credit report from all agencies every year. If you have not gotten one from them, you can pull it for free. There is a website you use to get them all at once - a legit site - not one that makes you buy a service. Just google it and I believe its the third link.
Second - There is no set rule that says "Sellers of brand X use ___ reporting agency". Dealers will make that decision on a store level. So it may be any one of the three
Lastly, dealers have a scoring system that is weighted to auto credit. The credit score you get from a reporting agency will be your overall score. However, dealers also see a second number called your auto score. That rating is based on teh amount, number and repayment history of car loans. It is always lower than the global score, but if you are steady on car notes, it wont be much different. If you are overall decent, but have had lapses in your car loans, then it will significantly affect that number.
Get your free reports from the agencies and start there. If you want the link, I can send in email. Again, its the legitmate credit site, not some bogus "sign up for alerts" thing.
Good luck
