It depends. You said you used a home office for three years, but never claimed it before. Does that mean you never claimed any home office expense? Or never claimed depreciation?
You have three years after the filing date to amend a return and claim a refund. If you never claimed a home office, you can amend your 2017, 2018 and 2019 returns and add home office expense.
If you previously claimed home office expenses but not depreciation, then use the original start date. You would lose the earlier depreciation deductions unless you file amended returns.
If this is the first year you are claiming home office expenses, you can use 2020 as your start date.
You do not have to claim depreciation if you use the Simplified Option for Home Office Deduction.