Congress is working on a stimulus bill that would authorize a third round of stimulus checks. If that bill is passed, the next round of payments would be an advance payment of a recovery rebate credit – just like the first two stimulus check payments. So, as it stands right now, your third stimulus check won't be taxable.
The same type of recovery rebate credit allowed for the 2020 tax year would be available for 2021 returns, as well. If you don't receive a third stimulus check, or receive less than what you're entitled to, you might be able to claim a credit for the amount you're owed on your 2021 tax return.