Jim Foltz's 'simple_shell' will work on a manifold [solid] form [needed for 3d printing anyway?] - however, the wall thickness of a 3mm shell isn't exactly 3mm all round - it might do though [it uses scaling about a center]...
Fredo's JPP [JointPushPull] has been recently updated to give a more constant thickness 'shell' - however, an exact constant shell thickness for multiple facets at a vertex is impossible to calculate...
Note that you might have problems making very small faces [<~1mm] so scale x10 and then do the 'shell', then scale down x0.1 before exporting [small faces can exist but making them is the trip-hazard...]
W