I have one of those Impalas. These are some of the best cars ever made, especially the 3.8L engine models!
Mine has a 3.8L engine and I have changed the coolant (and thermostat) on it. I can offer some DIY advice, but it sounds like you’re going to pay to have it done.
That car came filled with Dex-Cool coolant. It’s (red/pink). That green stuff was put in by somebody. It’s subject of great debate and that green/yellow coolant will work, however I’ve run Dex-Cool for over 300,000 miles. DO NOT mix them (green and red).
These systems vent the coolant reservoir to outside air and some loss to evaporation is normal. I’m not sure how much “less then a full gallon” actually is. A gallon in 3 years sounds a little excessive, but half a gallon not so much, though.
You need to change the coolant. If it’s thoroughly flushed out you could use Dex-Cool, but you probably will be Okay to use the Yellow universal coolant in this one. I’m not sure the green stuff is even available any more.
Do you have the 3.8L (3800) or 3.4L (3400) engine?