Is there any tools or functions which can run under UNIX system to draw a 3d letter?
Easiest is to model and export a set of 3D
meshes for all letters (a 3D font, if you
will) somewhere, and then use that.
Harder, but probably more like what you want,
is using a library like FreeType to extract
spline/outline data for a TrueType font, and
use extrusion/subdivision to actually render
it in 3D.