Re: 3 edged (of a octogon shape) diagonal stringer help
exactly as spruce mentions, any time i get into a deck with such a detail i break every thing down into simple "steps" pardon the pun
by doing so it makes things easier to complete and you can see things more clearly as you get further into it.
do the standard stringer first for each section, from there run a straight edge through and draw layout lines, this will create a template for which you can take measurements from to create the diagonal stringer. just note because its on an angle, the tread can become wider depending on how wide the stair itself will be which means going from 2x10 to a 2x12 to maintain correct "effective depth" which means the amount of material left on the stringer stock after the notch has been made.
other wise the other option is to switch to building small platforms to build the stair
fire up the saw and make some dust