Can a person go back to previous years (i.e. 3 years ago) and and claim leftover orthodontic amounts that were not otherwise reimbursed?

Yes, you can request an amendment to your past return as seen in our TurboTax article How Can I Amend My Tax Return?

You can also make an adjustment as per the Canada Revenue Agency (CRA): Change my return: Online adjustments for income tax and benefits returns

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