1) Your flight from Glasgow to Amsterdam is not on Delta as it does not fly this route. It is probably a codeshare operated by some other carrier. My guess is KLM. (Delta only operates flights at GLA to and from the U.S. )
2) You need to find out what time the airline desk opens up. If you are checking luggage, then you will have to go to the desk. If you just have carry on, print your boarding pass before heading to the airport and go directly to security. Glasgow is not that big of an airport and three hours seems like a lot. (I have never purposely shown up at an airport three hours ahead of time with just carry on.)
If it is KLM, the desk opens two hours before departure. So getting there three hours ahead of time will have you sitting for an hour.
Have you thought of how you are getting from your hotel to the airport.