I wanted to make a tutorial on how 3D objects can be designed in Photoshop but I never got a chance , now I think it's the time to go for it 
here's a little guide which can be used to achieve a 3D cube shown below:
Create a 200*200 white background document , make a fine gray square (color=7F7F7F) by selection tool on a new layer say Face1 , with this layer selected , press Ctrl+T to activate the transformation view right-click on square and select 'Skew' and skew the squre to get something like this:
Now copy the Face 1 and name the copied layer as Face 2 , give it a lighter shade of gray color (example: BFBFBF)and flip it horizontally , move it to the other side of the canvas to obtain an image like this:
Now we have two faces of the cube , to obtained the third face copt both the Face 1 and Face 2 layers and merge them into a single layer and rename it as Face 3 , give it the darkest gray shade (525252) of the two faces and flip it vertically , place this Face 3 layer below the first two layer in the Layer Palette , move the merged layer up so that the upper-side corners of this layer coincide with that of the Face1 and Face2 layers , you should get an image exactly like this one:
The last thing to do is to transform the Face 3 layer a little bit , select the said layer and go to 'Edit->Transform->Distort' now drag the mid-upper point of transformation down and then press Enter , move the Face 3 layer at tight position to achieve a similar result to this image:
These were the steps to achieve the a basic shape of a 3D-cube in Photoshop , after that the choice of colors and addons is up to you
The PSD file can be downloaded from here
I hope it helped you 
waiting for your results 
I'd make the 3D cube with the pen tool, you can use the selection tool, then fill with white and reduce opacity for the glossy look. If you don't understand, please leave a message. 
