Arrival Time at US Airport/Baggage Transfer in Amsterdam?

1) (3 hours before flight) - The airline sets the time limit. Assuming your travel agent has got that figure from the airline and not just made it up. But you could check the airline's website.

2) If you have a through ticket your bags will be checked through to Prague, which is where you will next see them.
You will go through immigration (passport check) at Amsterdam as that is where you enter the Schengen Area. No passport control at Prague, just pick up your bags and exit via the "nothing to declare" door.

Three hours sounds excessive to me too to arrive that early at SeaTac. However, keep in mind that you may need to check in with the gate agent again with your passports once you get to the gate. So I'd get to the gate ASAP. And you need a little more time to board an international flight than a domestic flight. I'd want to be at the gate absolute minimum of an hour before departure, probably earlier - they might start boarding an hour before.

You can check online a few days before for the departure status of that exact flight; see what time they start boarding, if you can find that online; the status of the flight should change to "boarding" when they start.

It’s pretty common to see a suggested arrival time of 3 hours for an international flight.

I suppose you could shave a half hour or so off if you have access to the priority airport screening line. These are generally available if you are flying in the premium cabin or through signing up for TSA Pre-Check or Global Entry. The latter two require that you fly on a participating airline. Link to airlines

https://www.tsa.gov/precheck/participating-airlines

But really why take the chance to save an hour? If it takes less time then enjoy a drink, meal or coffee post security. Better to be relaxed in the terminal than sweating it out in line or traffic.