Can a 3-inch toilet drain be offset vertically just below the flange? See attached sketch. I am blocked from moving the horizontal portion of the line by an existing duct (both of which run below the floor joists.

Yes, if you can get both ells in that distance. If not there are street ells that would take less vertical distance. If not there is also off set toilet flanges'

As Hkstroud said, yes, you are all set as long as the distance from the top of the pipe to the flange does NOT exceed 24" and you use the 45 degree fittings you put up in your drawing...;)

Mark

Thanks hkstroud and massplumber. massplumber, not sure what distance you mean can't exceed 24". From the top of the horizontal run to the flange?

Yes, from the top of the horizontal pipe to the flange...;)