I have 3 hours to kill on a layover. What is worth seeing?

Airport bar.

Nothing. The Air Canada lounge is nice if you are on star alliance ticket. But it is in the other terminal but there is a back way to get between it and the US terminal.

Also YYC security is woefully inefficient, so I wouldnt risk leaving airport, because those lines can be stupid long or completely empty, depends on the time.

Agree that 3 hours is not enough time. But if you end up having say 5 hours, then I would take a taxi to the Sheraton Eau Claire and walk around the Prince Island park.