Cantref Reservoir is the middle of the three reservoirs in the Taff Fawr valley in Wales. It is owned by Welsh Water. It is located in the Brecon Beacons National Park, mostly in the Powys unitary authority area and within the historic county boundaries of Breconshire. Part of the south west corner is in the Rhondda Cynon Taff unitary authority area.
There are more than three reservoirs in the Taff Fawr valley.
The context mentions a reservoir being one of three in the valley. Perhaps "more than three" was too vague for the AI, given the context, as it certainly doesn't say there are ONLY three.