I never received the first or second stimulus checks but yet it allowed me to claim that i never received the 3rd one. how can i claim the first two?

The only way to get the 1st and/or 2nd stimulus is to use the recovery rebate credit on a 2020 tax return.   The credit is in Federal Review  and it is based on your 2020 information.   

Did you file a 2020 tax return?   If so, check line 30 to see if there was an amount there.   If you did not get the 1st or 2nd EIP payment you either need to file a 2020 return or amend the 2020 return you filed.

You would have to amend the 2020 tax return that you filed to get the Recovery Rebate Credit for the two stimulus payments that were available in 2020 if they were not received.

But first you need to check if the IRS sent those payments to you so check your tax account - https://www.irs.gov/payments/your-online-account

If your tax account shows that the IRS sent the payments you need to have the IRS start a trace - https://www.irs.gov/newsroom/questions-and-answers-about-the-first-economic-impact-payment-topic-f-payment-issued-but-lost-stolen-destroyed-or-not-received

See this TurboTax support FAQ for amending a 2020 tax return - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/00/27439

You claim the first and second stimulus a Recovery Rebate Credit on your 2020 tax return. Please see How do I prepare a 2020 tax return? or How do I amend my 2020 return?

To be eligible for the Recovery Rebate Credit on your 2020 tax return you need to be a U.S. citizen or U.S. resident alien in 2020 and have a Social Security number that is valid for employment.

You cannot be claimed as a dependent by someone else to be eligible. 

Your credit amount will be reduced if your 2020 adjusted gross income (AGI) is more than: 

• $150,000 if married and filing a joint return or filing as a qualifying widow or widower
• $112,500 if filing as head of household or
• $75,000 for eligible individuals filing as a single or as married filing separately.  

Your payment will be reduced by 5% of the amount by which your AGI exceeds the applicable threshold above. (IRS FAQ Recovery Rebate Credit )

To claim the Recovery Rebate Credit in TurboTax, please follow these steps:

1. After entering all your information click "Search" at the top right and type "Stimulus".
2.  Follow the jump to link (if you do not qualify for the Recovery Rebate Credit the stimulus questions will not come up). 
3. On the screen "Let's double-check the amount you received" you will be asked about the amount of third stimulus payment you received. TurboTax will then calculate the amount of stimulus payment remaining that you are still entitled to get. 
4. Any stimulus amount remaining due to you will show as The Recovery Rebate Credit on line 30 of Form 1040.