Didn't file taxes for 3 years, now IRS won't provide refund?

Ok, not surprised, but what is the general procedure for fixing this?

I lived off of savings to take time to work on a personal project. So, I just didn't file for nearly 3 years.

Now I filed for 2018 but the IRS says my AGI stated for 2017 doesn't match their records.
Update: I know I should seek professional tax advice, but afraid they may charge a lot. Just wondering what the scenario is for something like this. I have legitimate return of $3400 for 2018, but I realize I may owe some fines for not filing the other years?
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