It says I should have received a third payment between March and May, of $2,800, however, I only received a payment of $600 in January

You would have to amend the 2020 tax return that you filed to get the Recovery Rebate Credit for the stimulus payment that was available in 2020 if it was 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-p...

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

The $600 was for the 2nd Stimulus payment from 2020 for 1 person.  Did you get all the 1st and 2nd Stimulus payments?   Did you have any more listed on your 2020 tax return line 30?   

The first 2 Stimulus payment only get reconciled on your 2020 return.   You don't report them on your 2021 return even though you got the second one in January 2021.

They sent a third Stimulus payment out starting in March 2021.  That was for 1,400 per person.   If you didn't get the 3rd one you can claim it on your 2021 tax return line 30.  

How to claim the 3rd Stimulus payment Recovery rebate credit on your 2021 return

https://ttlc.intuit.com/community/tax-credits-and-deductions/help/how-do-i-claim-the-recovery-rebate-credit/00/1855581