How to darken (or lighten) the inner part of 3D text?

Here a link to a step by step tutorial to 3D Text extrusion:

https://www.webucator.com/how-to/how-create-3d-extruded-text-adobe-photoshop.cfm

Hi
Assuming you have that as 3D text to adjust and re-render, you can adjust the lighting strength and position (don't forget the image based light in the Environment properties) , and also the extrusion material (Diffuse property) of the text object.
Dave