Join Date: Jul 2007
Location: San Francisco/Sydney
Programs: UA 1K/MM, Hilton Diamond, Marriott Something, IHG Gold, Hertz PC, Avis PC
Posts: 8,156
As others have said, you can simply show the TK card to get access.
However presuming you haven't checked in yet you can also change the FF number on the booking to the TK one. You likely can't do this online (you can't on the UA side, but might be able to on the A3 side), but UA will be able to do it over the phone or at the airport as long as you haven't checked in yet.
This won't have any impact on the "payment" for the flights (ie, the A3 miles), nor will it impact earning (as there isn't any for a reward flight!) but will give full *G benefits.