If TT says me and hubby are eligible for a 3rd stim pmnt of $2800, and we each rec'd Letter 6475 stating we rec'd $2100, then do I only enter $2100 or $4200 in the box?

For married individuals filing a joint return, each spouse will receive their own letter showing half of the total amount. So you would post $4,200.

See IRS statement:  Understanding Your Letter 6475

The only Stimulus issued last year was a maximum  $1400 per person. Not $2100 per person...I've not heard of anyone receiving less. How did you get your stimulus benefit? If it was by Direct Deposit, those payments were sent out between 3/1/21 and 5/1/21. Check out your bank statements to see how much you received. There were NO separate checks checks sent out for joint filers. Hope this helps