The Third Pinnacle is the sum of the first two, reduced. It uses no part of the birth date that the other two have not already used, and it could be written down without knowing anything about the person beyond the two figures already on the chart.
That makes it unlike almost everything else in the system. Every other position reaches back to an input, whether a month, a day, a year or a set of letters. This one reaches back only to other positions. In the language of measurement it is a derived quantity, and a derived quantity cannot tell you anything its sources did not.
The tradition does not present it that way. The third pinnacle is described as the height of a life, the stretch in which whatever the first two established comes to whatever it is going to come to, and it is given a character of its own with its own guidance attached. Read as a claim about a distinct phase, that is a real assertion. Read as arithmetic, it says that the middle years combine what came before them, which is a statement about addition rather than about a life.
The honest description is that the third pinnacle is a way of expressing the relation between the first two, and it is genuinely useful for that. A first pinnacle of four and a second of five gives a third of nine, and a practitioner reading the three together is reading a shape.
What it is not is an independent observation.