<human>: Can you condense the text into three sentences or less?

Good morning Reddit, long-time lurker here, who seems to have found himself in a predicament.

In 2014 I started a seasonal job, March through June, and I returned to that job in 2015, and again in 2016(Though this year it extended to September). It was my first job, so I knew nothing about taxes back then(And only barely know anything now). I didn't file taxes or tax returns for 2014 or 15, because it seemed to be taken automatically out of the pay and I was told I didn't have to pay anything if I made less than 10,000, which I did until 2016.

I did/do look at every pay stub I receive. But I'd never shared it with anyone, nor had I ever looked at anyone else's, so I didn't realize there was a mistake until my sister(Who works for the same company) and I received our final paychecks this year, and for an unrelated reason, compared stubs for the first time. We were rather shocked to find that she payed over double what I did in taxes, despite earning just over half what I did(She's part time, I'm full). That's when I noticed I was exempt from State and Federal Withholding, and she was not. After some research, I found that I should not be either.

I don't know if this was my screwup or my boss' screwup, and I don't care, it was three years ago. I did some googling, and found that other folks have made the same mistake, but I'm posting here anyway because I've not found anyone that worked for so long before noticing. I guess I'm just a special kind of clueless.

Having waited so long, I'm a little worried the IRS is going to hit me with a fine. I have the savings necessary to pay what should have been withheld, barring a fine or massive interest, so my ultimate question is, what's my next move? Do I need to do anything before filing taxes(Which I've never done before) to let them know there was a mistake, and I'm not intentionally evading taxes?
<bot>: Worked a seasonal job for 3 years, never filed taxes, just now realized that I've been exempt from Federal and State Withholding all this time. What do?