It should automatically give you the 1,400 stimulus if you qualify. When you go though the questions it will ask if you got 4200 for the 3rd Stimulus (1400 for 3 of you). You say NO and enter 2800 you did get. Then 1400 will be on 1040 line 30 for the Recovery Rebate Credit.
To get to the stimulus questions in TurboTax at any time, you can search for stimulus or recovery rebate credit then hit ENTER and there should be a Jump to link
If you are trying to trigger it,
Click on Other Tax Situations at top
Click on Let's get started
Answer some questions then Continue
Then click on Let's keep going
Next screen is for the Third Stimulus, if you qualify
