[Zigûr]

Does Professor Tolkien ever specifically use the phrase "Rings of Power" to include the Three?

I suppose it's implied but we could just as easily argue that "Rings of Power" simply means "the Great Rings and the One Ring".

In fact a search of The Letters and The Lord of the Rings itself suggests to me that "Rings of Power" is mostly used to refer specifically to the Great Rings (particularly the Nine) and "the Ring of Power" (singular) is mostly used to refer specifically to the One Ring.

In the Tale of Years, Professor Tolkien distinguishes the forging of the "Rings of Power" from the forging of the "Three Rings".

Meanwhile in "Of the Rings of Power and the Third Age" in The Silmarillion the Three seem to be more included with the "Rings of Power": the Great Rings are described as "all the remaining Rings of Power", which is to say those Sauron found after Celebrimbor hid the Three.

So I would argue that it's simply not a very specific term. Sometimes it seems to encompass the Great Rings, sometimes it's the Great Rings and the One Ring, and sometimes it's the Great Rings, the One Ring and the Three Rings.

As a result, Gandalf isn't being completely accurate, but I'd argue that he isn't entirely wrong either. In any event it was forbidden to speak of the Three and not relevant in any case.