Do all toilets set 12" from the wall to the flange bolts? How much fall is needed for the 3"drain to the septic tank connection?

The most common toilet is built for a 12" rough in from finished wall, however 11" and even 10" rough in toilets are available. Drain lines should have a slope of 1/8 to 1/4" per foot. Not less, things don't drain. Not more, water runs away leaving solids behind.

3" pipe should slope at 1/4" per foot. 4" is 1/8" per foot.

As Harold correctly states toilet setting measurement varies. Manufacturers recommend 12". We routinely rough-in our toilets at 11 1/2" away from finished wall and still have 1" space between tank and wall.

The above answers are right on the money. If you were in a bind there are even a few 9" toilets but 12" is the most common. You must also be 15" on center off any side walls to be within code.