Kathleen, good choice choosing YEG for your 3-hour layover! And Yellowknife does have a rugged beauty, especially during the hot summer months with nearly 20 hours daylight at Summer Solstice. Be sure to visit the Wildcat Cafe in old town.
At YEG, after your 2 hour flight from Yellowknife, you'll be able to easily pre-clear US Customs before your return flight to the US (assuming that you have a direct flight). If you want to relax, shop or dine at the airport, there are plenty of options to keep you entertained. Here's an airport terminal map:
http://flyeia.com/sites/default/files/files/shopping_and_dining/airportterminalmap_shopdine2017.pdf
Try a super-healthy Booster Juice (invented in Edmonton!), an iconic Tim Horton's coffee, or stop in at Gretzky's Wine + Whiskey for food and/or beverages (if you're not a hockey fan, Tim Horton and Wayne Gretzky both became famous playing NHL hockey). Discover Alberta and the HBC Trading Post are two of the many retail shops with iconic Canadian and local souvenirs or gift items. Check out the Living Wall and some of the other public art throughout the terminals. The Northern Lights Airport Volunteers (in their colorful orange shirts) provide a friendly welcome and you might encounter pet therapy volunteers with their dogs, cats, or rabbits. Some of Edmonton's festival entertainers also perform during busy times at the airport.
If you had more layover time, you might even play a round of golf at the on-site Redtail Landing Golf Course! Depending on when your trip is, the Premium Outlet Collection mall at the airport will open later in 2017 with 100 more shops. (Full disclosure: I don't work at the airport but have spent plenty of time there over the years as it's my home airport.) Oh, and there's lot more to see in Edmonton than just the airport ... enjoy your trip to the spectacular NWT.