Change the Render Settings
Select Scene on the 3D Panel and click Render Settings, use these: Face Style => Ray Traced, Enable Line Rendering => Checked, Edge Style => Constant, Crease Threshold => 2.

Adjust the Lights
Go to 3D Panel and select the Infinite Light 2. Set Intensity to 0,5 and set Color to #9999aa. Uncheck Create Shadows. Now select Infinite Light 1. Set Intensity to 2, click the Rotate the Light Button, click and drag on the canvas to rotate the light to get some nice shadows.

Edit the Diffuse Texture
In the Layers Panel, double click the 3D Diffuse Texture of the "3D" layer, this will open up the layered texture document in a new window. Apply a Gradient Overlay to "Mosaic" layer using these settings: Blend Mode => Overlay, Gradient => Blue, Red, Yellow.

Edit the Diffuse Texture
Now create a new layer on top, get the Gradient Tool, pick Spectrum gradient and set Style to Radial. Fill the layer diagonally as in the below image. Go to Filter > Blur > Gaussian Blur and apply with a Radius of 100 pixels. Then Alt-click the line between "Layer 1" and "3D" layers to mask the rainbow. Set Layer Opacity to 80%.

Fix the Edges
On the perpendicular faces, a few pixels of diffuse image is streched along the face and looks disturbing. To fix this, we’ll expand the border of the text outwards by applying an Outer Glow Layer Style to the "3D" text layer. Use these settings: Blend Mode => Normal, Opacity => 75%, Color => #d2c6bd, Spread => 29% and Size => 5px. We’ll also apply an Inner Shadow to darken the edges slightly using these settings: Opacity => 50%, Size => 45px.

Fix the Edges
We have the same problem on the top and left far perpendicular faces. To fix these, go to "Border" layer and double click the Stroke Layer Style and change size to 82.

Create the Self Illumination Texture
Now we’ll create a Self Illumination texture and lighter pixels in this image will be self illuminated on our 3d model. First go back to "Awesome Blocks" document and double click the "3D Depth" texture to open it in a new window. We’ll not change anything on this texture but we’ll use it as a template to create the self illumination one. Go to Image > Duplicate Image and name it "Self Illumination".

Edit the Self Illumination Texture
Delete the "Border copy" and "Border" layers and fill the "Mosaic" layer with Black.

Edit the Self Illumination Texture
Apply an Inner Shadow Layer Style to "3D" layer using these settings: Opacity => 100, Choke => 25%, Size => 55px. Set Layer Opacity to 35%. Turn off the Stroke Layer Style.

Edit the Self Illumination Texture – Put Some Lights
Create a new layer named "Lights", get the Elliptical Marquee Tool and select a little circle and fill it with #6d6d6d. Place the duplicates of this circle regularly on the text, then select all circle layers and merge them. Apply a Gaussian Blur filter with a Radius of 7 pixels and apply an Outer Glow Layer Style and use these settings: Color => White, Opacity => 10% and Size => 5px. Now save this document as "Self Illumination.psb".

Assign the Self Illumination Texture
Now go back to "Awesome Blocks" document, go to 3D Panel, select the 3D Material. Click the Edit Self Illumination Texture button and choose Load Texture and load the "Self Illumination.psb" file.

Some More Render Settings
Select the 3D Material in the 3D Panel and set Reflectivity to 25 to get some reflections. Select the Scene in the 3D Panel now and set Anti-Alias to Best to achieve a smoother result. If you are planning to play around with textures and lights, i suggest you to set that to Draft, otherwise it may take too long for Photoshop to update the changes you made.

Congratulations
It’s almost done. Now go to 3D > Rasterize to convert the 3D layer into a regular layer. Then get the Polygonal Lasso Tool, select the faces outside the thick 3d border and clear. Below is my final image. I hope you liked this tutorial and had fun making it.