We did not sell any stock, just exercised option. Is it necessary to amend return to enter info from the 3921?
A Form 3921 is issued when you exercise an ISO. Any stock purchased via an ISO that is not sold by year end is an AMT adjustment item. So I'd say "yes" you might need to amend as you said you did not sell the stock acquired via the ISO exercise.
Enter the Form 3921 information using the "ISO Exercise and Hold" interview. That will make the AMT adjustment. If you are not subject to AMT your tax liability will not change and no amendment would be necessary.