Three hours is the threshold, but three hours measured how? The Court of Justice answered this in Germanwings v Henning (C-452/13). Arrival time is the moment at least one of the aircraft doors is opened and passengers are permitted to leave.
It is not the wheels touching the runway. It is not the aircraft reaching the stand. It is not the time the airline recorded for its own punctuality statistics, which tends to be the earliest of the three.
Because the gap between those moments is routinely fifteen to thirty minutes, and compensation is all or nothing at the three hour line. A flight the airline logs as two hours fifty five late can easily be three hours ten by the time the door opens.
That is the difference between nothing and up to €600 per passenger, and it is exactly the sort of claim airlines reject on the first pass.
If the airline refuses on timing, ask them to state the exact door open time and evidence it. The burden of proof is on them, and a lot of borderline refusals do not survive the question.
It is the arrival at your final destination, not at the airport where things went wrong. A forty minute delay on the first leg that makes you miss a connection and arrive eight hours late is an eight hour delay, provided the whole journey was on one booking.
Work out your delay