If you think your prior year returns may be incorrect, you can certainly go through the Amend process. Be sure to save a copy of your originally filed returns first.
Click this link for instructions on How to Amend a Prior Year Return.
If you look at Form 1040 for those years, you can see that there are several other entries after your AGI, including your Standard or Itemized Deduction amount, to arrive at your Taxable Income.
Check that all tax withheld is correct (Total Payments) and go through any topics in the Deductions & Credits section that could apply to you.
If you had different employers during those years, the difference may be in the amount of tax they withheld from your pay. You may want to submit an updated Form W-4 to increase your withholding.
Click this link for more info on Why is My Refund Less This Year?
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