I need to add a dependent for my last three years returns, 2021-2023, which do I amend first?

When adding a dependent it does not matter what order you amend the returns.

See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-already-filed/L4VjJ9BA2_US_en_US?uid=ld1n99kr