Firstly there is only 1 lounge at AGP (full stop!) and I'm certain there is no way you will be be able to reach it (it is in the departure lounge).

Also AGP airport is very limited for shops (given its size) both airside and landside.

However 3 hours is not actually THAT long.

Assuming this is during daytime hours there is something you could checkout - I've never had time - The (Art) Gallery! Its half way down the ramp that connects the main terminal checkin concourse to landside restaurants and additional checkin area.

But if thats not open or proves limited then I'd just go an get a beer from the bar next to burger king, and sit back watching the airfield activity out the window.

Seriously, if you have a hire car or anything to sort out, 3 hours will fly by.

N.b. there is a LOT of construction activity at present, the airport is undergoing major expansion.

EDITED TO ADD: Additionally, you simply would not want to get to this lounge!