One thing I'm not clear on. You say you have 3 months of credit history, but are disputing something that's 6 years old? I'm not sure how that can be. Unless you're saying that you're disputing that item on the basis that it's 6 years old and shouldn't exist since you have 3 months of credit history?
I have no feedback unfortunately on the Amex card that you're interested in. I think in general, Amex likes to see more than 6 months of credit history. I'd suggest hitting up the credit pulls database and searching for that card. See in the comments section anyone that was either approved with short credit history or denied with it. That should give you a little insight as to what you can expect.