3 hours is maybe just a little too short to leave the airport, unfortunately. MSP is pretty big to navigate - the layout is bizarre, kind of like a giant box, so going from one corner to the other is burdensome. And the security checkpoints can back up (although CLEAR probably helps some with that.)
At four hours, though, I'd say going to the Mall of America is feasible. It's a 13 minute ride that departs every 10 minutes by light rail each way, so that's ~50 minutes total in transit. Add 30 mins to navigate MSP, 20 mins for security and 40 mins before your flight to be present for boarding and you get 2h20min spoken for of your 3 hour layover. The remaining 40 isn't enough to do much meaningful with, but 1h40 would be...