How can i get my 3rd stimulus check I'm on SSDI didn't get it 1400.00

$1,400 is the 3rd Stimulus check.  The 3rd Stimulus payment started going out in March 2021 and is for 1,400 for each person listed on your tax return, no age limit.  That was on your 2021 return.  If you didn't file 2021 you can still file for it.  It goes on 1040 line 30 as the Recovery Rebate Credit.  

To do a prior year return you have to buy the Desktop program here,
https://turbotax.intuit.com/personal-taxes/past-years-products
You will need a full Windows or Mac to install it on.

If you want to file for free you can fill out the forms by hand. Here are some basic forms…..

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

You have to print and mail prior year returns. When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS received the return. Don’t forget state.