Okay first open up your image that you want to add a 3d brush too. I will just be using a blank white canvas

Okay go up to layers>new raster layer and match your setting to mine



Okay now stamp your brush where ever you want it
Okay now select your square selection tool, and highlight over your brush!
You now should have a blinking square border around your brush!
Okay now click INSIDE of the blinking border (on your brush)
Now just your brush should have a blinking border!
Go up to effects>3d effects>inner bevel, and match your setting to mine



Once you have matched your setting to mine hit "okay"

Now your brush should look sorta 3d! Bellow is how mine looks!