A 9D pipe bend is where the bend radius is 9 times the pipe nominal diameter.
So as you specified a 3 inch diameter pipe, the radius of the bend would be 27 inches or 685.8mm

Isn't this thread also in the other metallurgy forum?

Also, is't a 3" SCH80 pipe outside diameter 88.9 mm (3 1/2"), thus making a 9D bend: r = 800 mm ?

Thanks. My query is on whether we should consider NPS or OD in the equation Radius = 9 Diameter.

I have always understood the standard to be the nominal diameter - like this:

Pipe bends are classified according to the centerline radius (CLR) of the bend as a ratio to the nominal pipe diameter.
For example, 4″ N.P.S. pipe which is bent on a 6″ CLR is classified as a 1½D Bend (1½ times the nominal pipe diameter).
When bent on a 12″ CLR, the bend is classified as 3D.
http://apexpiping.com/technical/

However a bit of research does suggest that some people use the outside diameter ?

In tubing we always use the OD, and in pipe the NPS. Just one of those things.

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P.E. Metallurgy

For inch-size piping, I have always seen and used the bend radius to be at the center of the elbow and the radius a multiple of the nominal size, not the actual size. For instance, short radius 90 degree elbows are 1.0R and long-radius elbows are 1.5R (check any fitting catalog). In this case a 4" short radius elbow has a radius of 4" and a long radius elbow has a radius of 6". If these were called out as "D" bends they would be 2D and 3D respectively. For a non-standard elbow, it is normally specified as a multiple of the diameter, not radius. So a 4" 6D elbow would have a 24" diameter or 12" radius. The pipe is 4 1/2" OD but that does not matter, it is the NPS (Nominal Pipe Size) that is used. The convention of using the pipe centerline makes it very easy to make (and check) single-line drawings. In the image below, "A" is the bend radius.