Start a new document by pressing Ctrl + N. I used 500 X 500 (you can make it any size), than go to Filter -> Render -> Clouds. 3D Blocks Background

Now go to Filter -> Pixelate -> Mezzotint and use the following settings. 3D Blocks Background

After that, go to Filter -> Blur -> Radial Blur and apply these settings. 3D Blocks Background

Once that's completed, go to Filter -> Stylize -> Extrude and use these settings. 3D Blocks Background

Lastly, go to Hue/Saturation by pressing Ctrl + U and use the following settings, you can play around with these if you want other colors. 3D Blocks Background

This is how your finished background should look. 3D Blocks Background

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