We did not get stimulus for 2020. Turbotax deducted $3600 on line 30 from what we owed fed. Today we received a $4200 check from the irs. Do I call? Return check?

OK ... first the balance due on the return ... you still owe it if you have not yet paid it.  

Next ... the third stimulus ... $1400 x 3 people on the return = $4200 which could not be taken for a balance due on the 2020 return so you can either cash the check and use it to pay the outstanding balance due you still owe or not.  However the longer the outstanding balance due is left unpaid the more it will cost you in penalties and interest  so if I were you I would use the free money from the IRS wisely. 

Oh we paid what we owed for fed and state upon submission of taxes.  Did not know there was another $1400 per person.  Im surprised!!  Good news then!!!  Thank you!