Customs at first EU country or final? How is AMS?

It's not customs you need to worry about.

• Customs is the check on goods. If the contents of your luggage contains anything that is banned or on which they need to charge import duties. This happens when you finally retrieve your bags at your final airport. Customs is a random check only. You just walk through the green "nothing to declare" door, 99% of the people aren't stopped, so it takes zero time.

• Immigration (passport control) is the check on people. Whether you are allowed into the country and for how long. This is what can take 1 hour or more.

It's not the EU that matters, it is the Schengen Area.

The Schengen Area is a Passport Union, you get your passport checked when you first enter the Schengen Area, and not again until you leave.
But, the UK is not part of the Schengen Area. So you will not need to enter the Schengen Area at Amsterdam. If you are coming from outside the Schengen Area you will stay "airside" (no immigration), and UK immigration/passport control will happen when you land in Scotland.
Amsterdam Schiphol is a very efficient airport, you will be sitting around for most of your 3 hours.

Clear?

As far as I can see the poster is flying in from outside Europe, no need to exit the Schengen Area as they never enter it.

We fly Delta the majority of time. We have flown Delta from the USA to AMS with just over an hour layover more than once and always have made our connecting flight. Of course our initial flight left on time! The employees at AMS are very helpful. We have been placed in a shorter lane to get through immigration quicker. Someone has always been at the immigration line checking on travelers who have a short connection time. Years ago, due to a very tight connection, an employee actually met me at the gate and walked me through to the next flight - amazing service! Recently we had a three hour lay over at CDG and our flight arrived two hours late. While we made our flight by a few minutes it was due to running through the airport not due to any help from the employees. I have confidence in AMS. Good luck!

https://insideflyer.nl/schiphol-central-security/

Transfer passengers

To separate passengers from cleared and non-cleared origin, new floors
have been built on top of the E, F and G piers. Depending on the
origin from the flight passengers will be directed to another floor.
If the aircraft is arriving from a checked country (Schengen, EU or
US) , then the entrance to the Lounge will be used as is the case in
the old situation. Without any hassle passengers can shop and dine of
go to the airline lounges. If passengers are arriving in Schiphol they
can walk freely to passport control. Passengers who need to transit
within Schengen have only to pass the mandatory passport check between
Lounge 1 and 2. The old security checkpoint between Lounge 1 and 2
will disappear.

If the aircraft is not from a clean country, passengers will exit the
plane on the new upperfloor. Transfer passengers will head to the
beginning of the G-terminal or the filter above the Holland Boulevard
for security and after this check they will arrive in the lounge to
continue to their connecting gate and enjoy Schiphol’s facilities.

Passengers that will take an onward flight to a Schengen country have
to pass transfer security first to enter the non-Schengen lounge.
After this check they are able to pass passport control in Lounge 2
and 1 and can walk to their gate in the Schengen area. The other way
round, Schengen passengers who are travelling to a non-Schengen
destination only need to pass passport control for their connecting
flight....

Just came back through AMS it was easy peasy. Your 3 hour layover is plenty. And we did experience the AMS staff calling the flights that were within 90 minutes to departure to move to the head of the line at passport control. AMS is so nice versus CDG - what a nightmare that airport is!!!

We have flown Delta via Schiphol. We usually have about 3.5 hour layover. It's our favorite layover. Despite the airport size, they know what their doing. So well organized. We seem to always have time to relax and walk around before departing. We are scheduled to connect with KLM again in a few months.

We recently changed planes at AMS, coming from the States on Delta and switching to KLM metal. The lines looked daunting, but they moved fairly quickly. Unless your first flight is horribly delayed, three hours should be sufficient at that airport.

Wow, you have gotten some good info and mostly bad.

When you arrive in Amsterdam, you will transfer and not have to go through immigration or customs. When you arrive in Scotland, you will go through immigration and customs.

Go to this website, enter your arrival and departure info and it will give you instructions.

https://www.schiphol.nl/en/transfers/

Just flew home on KLM through Amsterdam.....told the Delta folks we booked with that they did not give us enough time to change planes in Amsterdam and of course they denied that it would be a problem......we landed at 1:20 and had a connecting flight leaving for JFK at 2:40.......we raced to immigration, waited in a line at the passport queue and ran down past 3 gate corridors to our connecting flight and made it just as they were putting passengers on......if there had been any longer lines at the passport control we would not have made it......no one at Amsterdam seemed concerned that we might miss our flight......a guy behind us in line asked of he could get ahead of us so he would make his flight and then realized his was leaving the same time ours was.....appreciate the fact that some people have gotten help at Amsterdam and even been escorted through but we weren’t!