Hey,

LuaTeX currently supports only a very special kind of Type 3 fonts:
They are generated for traditional TeX bitmap fonts. While this used to
be the most common use case for Type3 fonts, the fully general concept
of these fonts has recently been reinvented under the name of SVG fonts
(and friends). While they can be implemented in LuaTeX to some degree
through virtual fonts/node processing etc., converting them into "real"
Type 3 fonts in the PDF file would give some important advantages: Such
fonts are called like other fonts and can get tounicode vectors,
allowing better accessibillity and better Copy&Paste behaviour. Also
viewers generally treat them as text instead of graphics, allowing
proper interaction with features of PDF viewers.
Beside from supporting SVG fonts, it also allows to use general MetaPost
fonts which might not be easily convertable into normal formats for vector
fonts.

As far as I can tell there used to be an implementation of general Type
3 fonts for pdfTeX which was dropped in LuaTeX, mostly because it was
never actually used. My suggested new implementation is a bit more
flexible by not requiring external files defining the font but instead
generating each font character based on individual node lists for every
glyph. One way to make this work would be to add a special `format`
named e.g. `node` (given that `type3` is already used to imply bitmap
fonts) and add a field `node`/`list`/`head` to the character table
referencing a node list which should become the font glyph.
The detaily like bounding box calculations etc. would be the
responsibility of the Lua code loading the font.