I am filing as self employed for a cash only business. Why does 1400 3rd stim credit not get added to my est. Return amount

When you are self-employed you have to pay self-employment tax yourself for Social Security and Medicare, as well as ordinary income tax.   Since you are not working for an employer, no tax is being withheld from your pay.   Income tax refunds come from paying more tax than you owe.   If you did not pay any income tax during the year--like by paying estimated quarterly tax payments---then the only "refund " you could get would be from refundable credits such as the recovery rebate credit or child tax credit or maybe earned income credit.  

And those credits are applied to any tax you owe....so you might not even receive the full amount of the recovery rebate credit  or child tax credit as a refund.

Are you new to being self-employed?

https://ttlc.intuit.com/community/self-employed/help/how-do-i-report-income-from-self-employment/00/26653

https://ttlc.intuit.com/community/self-employed/help/what-is-the-self-employment-tax/00/25922

https://ttlc.intuit.com/questions/2902389-why-am-i-paying-self-employment-tax

https://ttlc.intuit.com/questions/1901340-where-do-i-enter-schedule-c

https://ttlc.intuit.com/questions/3398950-what-self-employed-expenses-can-i-deduct

https://ttlc.intuit.com/questions/1901110-do-i-need-to-make-estimated-tax-payments-to-the-irs