Do you already have your flights booked? Where in Alaska? Fly into Anchorage? Juneau? Somewhere else?

Do you mean three nights (thus three full days) or three days/two nights? When do your flights arrive/depart? That makes all the difference in the world regarding how much you can plan.

With three days (depending on how you figure them) I would probably just go to either Homer or Seward. Personally I would pick Seward. I definitely would not go to both with your short time.

Take your time driving there and back - Girdwood, Alaska Wildlife Conservation Center, very scenic stops, maybe the Portage Glacier Visitor Center will be open.

Take a 6 hour or longer (NOT shorter) Kenai Fjords National Park boat tour. The shorter ones don't get out of Resurrection Bay so you don't see nearly as much wildlife and you would miss the glacier.

Right now the tunnel to Whittier is closed to cars and it is uncertain when it will reopen. So you would need to park the car on the Anchorage side of the tunnel and take the train to Whittier in order to take the Prince William Sound tour, and I am not sure of the train times - you might need to spend the night in Whittier one way or the other. In any case, I think it would be a very poor use of your precious three days.

If your three days is really three nights, and if you fly out later on the fourth day, taking a day trip to the Matsu Valley would be worthwhile. This is just north of Anchorage.

John