Is there any canonical evidence that those "three circles" on the front of the saucer are in fact forward sensors and not windows? Or any Matt Jefferies sketches that confirm this? I don't think we have come across any evidence that would support this. But, I could be wrong.
No, there is not. It's a conjectural idea of Franz Joseph who interpreted the three circular "lights" at the bow to be "search sensors and deflectors" on his Deck 7 plan, instead of windows. Matt Jefferies' schematic of the ship shows these "lights" but doesn't provide a description what these are.
It should be noted that the "pilot" VFX model had a position light in the center of the three (same for the triumvirate of the circular "lights" at the underside of the saucer).
IMHO these are viewing ports of the ship's reception room we saw in "Journey to Babel". To me it seems like the most suitable place to have a magnificient view of what lies ahead (a concept they reflected in ST V, though one could argue whether the room should have an ancient, wooden steering wheel for decoration or not