The IRS identified people who filed 2021 returns and did not use the recovery rebate credit so the IRS sent out those payments to them beginning in January 2025. If you could NOT be claimed as someone else's dependent for 2021 and did not receive the 3rd stimulus or put the recovery rebate credit on a 2021 return you can still do so if you file by April 15, 2025.
Look at your online account with the IRS to see if they sent you $1400 in 2021.
https://www.irs.gov/payments/your-online-account
If you could be claimed as someone else's dependent in 2021, you are not eligible for the payment.
If you were eligible and do not receive a payment soon from the IRS, the other way to get the payment is to r amend your 2021 return to use the recovery rebate credit before April 15, 2025. If you did not file a 2021 return at all, then prepare one and mail it in.
2021
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
Here is the IRS 2021 Form 1040
https://www.irs.gov/pub/irs-prior/f1040--2021.pdf
or if you want bigger type use 1040SR for Seniors,
https://www.irs.gov/pub/irs-prior/f1040s--2021.pdf
And 2021 Instructions
https://www.irs.gov/pub/irs-prior/i1040gi--2021.pdf
2021 Tax and EIC Tables
https://www.irs.gov/pub/irs-prior/i1040tt--2021.pdf
Sch 1 : https://www.irs.gov/pub/irs-prior/f1040s1--2021.pdf
Sch 2 : https://www.irs.gov/pub/irs-prior/f1040s2--2021.pdf
Sch 3 : https://www.irs.gov/pub/irs-prior/f1040s3--2021.pdf