EC/261 eligibility for a multi-flight itinerary only partly in the EU

I have booked an itinerary on Turkish Airways travelling from outside the EU to the EU via Istanbul, consisting of four flights, all on the same PNR.

Asia-IST
IST-EU
EU-IST
IST-Asia

If I'm parsing my Air passenger rights page right, since TK is a non-EU carrier, the only flight out of these four where EC/261 compensation rules apply is flight #3 ("your flight departs from the EU to a non-EU country operated by an EU or a non-EU airline"). A non-official site also states that:

In some cases, disrupted flights outside the EU may be eligible under EC261 if they connect to a covered flight that is with the same carrier and part of the same flight reservation (under one booking reference number).

But does not explain what these "some cases" are, much less point to an authoritative source.

In my case, TK has cancelled flight #1, effectively invalidating the entire itinerary. If TK does not notify me of flight #1's cancellation in time (at least 14 days), am I entitled to the EU compensation for flight #3?

Note that this is not a directly connecting flight, so I presume the answer is "no", but it would be nice to find an authoritative source.

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How do I file a EU261 claim?

To file a EU261 claim, you must have a valid ticket and booking confirmation. Although revenue and award tickets qualify for compensation, free or reduced fares that are not available to the public are excluded from compensation.

Is my flight eligible for compensation?

You're legally entitled to get compensation if the cancellation is the airline's responsibility and both the following apply: the replacement flight delays your arrival by 2 or more hours. your flight was cancelled less than 14 days before departure.

Does EU261 still apply to UK?

Although the UK formally left the European Union a year ago, air carriers will know that European Union Regulation (EC) 261/2004 on denied boarding, cancellation and long delay (EU261) has continued to have force in the UK throughout 2020 as though Brexit had never happened.

What is EU261 disruption compensation?

EC Regulation 261/2001 is an EU regulation. It protects passengers who've suffered a flight delay, cancellation or been denied boarding due to overbooking. Under this legislation, if your flight's eligible, an airline can pay up to $700 in compensation.

EU Air Passenger Rights

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: LT Chan, Andrea Piacquadio, Andrea Piacquadio, Andrea Piacquadio