3 hour layover -- any suggestions?

My advice would be to chill. After arrival, you will have to go through immigration if this is your first stop from the US. If you exit airside, you’ll also have to go through security to board your flight to Athens.

Three hours isn't that long, you'd need six to do anything outside of the airport

On my last layover, I chilled in the indoor/outdoor park above the D gates. You can get a bit of fresh air in the outdoor part, and charge your phone in the indoor lounge seats (complete with fake trees). It was a great place to kill some time with a cup of coffee.

https://www.iamexpat.nl/expat-info/dutch-expat-news/worlds-first-airport-park-schiphol

If this is Schipol, then you can view a smattering of items from the Rijksmuseum that has a small 'museum' or get your feet treated in a fish spa. I would check out the duty free for a nice selection of Dutch cheeses to add to your Greek pre dinner snack and drink. I am a sucker for their 'stroopwaffel'. Thin waffle cookies with caramel and spice filling.
These days my flights are rarely on time, so I wouldn't plan anything outside the terminal. But, if you need sunlight and want to stretch your legs in the parking lot or sidewalks, there used to be coin lockers inside security.
I like 'sleeping in airports' website for stuff to do or information that isn't always user friendly from the airport's own website.

I agree, three hours is not enough to leave the Secure zone. Maybe decline the less than satisfactory "snack" offered before arrival on the plane, then spend your time stretching your legs, looking at the shops, then find a place for a nice coffee and pastry or more of a meal. Your next chance to eat will likely be much later in Athens. I always found Schipol to be a nice airport to spend some time in.

The Rijksmuseum used to have a small display of Dutch masters at the airport if Art interests you, do not recall if it was within the secure area or outside of it, might be worth looking into.

3 hours is nowhere enough to consider leaving the airport, plus there really isn't anything just outside.
There is a little free Rijksmuseum display between lounges 2 and 3, and I think one of the largest "secure" shopping areas I've seen in any airport, so there's plenty to see and do.
It was already a 20-minute walk from security to our gate, so don't underestimate how large the place really is.

Is the flight to Athens on the same ticket? Are you checking any luggage? The answers to these questions may affect how much time you actually have between flights.

Having gone through Schiphol 4 times in the last 2 years, I agree that staying on the airside if possible is the best strategy.