I'd put this in the 'life's too short' category, and buy precut treated
stringers from the deck aisle at the Borg. At most, they will need a
little trimming to match the as-found conditions, and to make sure the
rise and tread depth are the same for all the steps.

Those "pre-made" stringers are fine provided they are the correct
riser and tread, in the OP's case it must be 7" step and 11" tread.

For 36" wide, I would use 3
stringers. Even with 2x6 treads, a 24" span in center will feel bouncy
for larger folks.

A 2X6 can safely span 24" with no problem. I have a 5 foot wide stairs
with 3 stringers and there is 25" between each stringer and I have no
problems with bounce