You won't notice a difference after 3 tanks, not unless your car is actually mapped for it.
The Fiesta is mapped for 95RON in this country, and although the knock sensor will cause the engine to compensate when a higher RON fuel is detected, it will only allow the engine to run correctly when using it, you won't see a higher gain.
If the car is mapped for it, or from the factory is designed to use high octane fuels, then you will make a difference.
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