What to do if I just found out my salary has been wrong?

I've been at my current employer for three years and I have just noticed that my salary has been wrong from day one. I know that I should have noticed this from the start but I just saw it. My employer does not typically offer an employment contract or any offer documents(very small company) so neither me nor my company have the original agreed upon amount written for proof either way. So my manager said I should decide what I want to have done and he will discuss it with me and propose a solution to the higher ups. If I had documentation this would amount to about a $5000 increase in pay for each year as well as the increased 401K contributions and matches and so forth. If we had proof I'd ask to split the difference and give me a lump sum of $7500 and just increase me to the proper level going forward and be done with it. Since we don't have proof either way I'm thinking I should just ask to be moved up to what I should be at going forward with no back pay or anything? What does everyone think about that or any other suggestion? Thanks