This is a complicated aspect of air transportation pricing.

In simplest terms, pricing is NOT the sum of the segments but rather determined by your origin and destination (O/D). So a S fare might be available on a segment for one O/D but not for another O/D -- even for the same segment. (I have probably overly simplified this, but that is my level of understanding).

You might have a chance to get the lower pricing by piecing together the series of segments by use of multiple destinations booking (is this your #3), but you may also get a higher price.

Other related issues --- married segments, captive airport / hidden city pricing ( AAA-BBB-CCC is cheaper the BBB-CCC)