: The advantage of two eyes is that it gives us a little bit more depth
: perception by expanding our cone of vision slightly to the left and right.
The rest of your thoughts about the Pfhor's third eye being useful for better peripheral vision are fine, but depth perception doesn't have anything to do with "cone of vision". Two eyes give us depth perception because the difference in angle between objects and each of the two eyes (parallax) can be used to gauge distance. The same effect could be accomplished by weaving side to side and comparing how things look at each end of the weave (closer things will appear to move more, distant things less), but two eyes lets us do that without actually moving our heads, as we can just see from two places at the same time.