I'll answer this in two parts :
Yes you can edit the content of a 3D postcard.
In the 3D panel click on the texture
In the properties panel click on the folder symbol next to Diffuse Material and choose Edit Texture
A psb file will open with you text which is editable
However - the more usual way to create 3D text is to highlight the text layer and choose 3D extrusion from selected layer (not 3D postcard). Then you will see the Edit Source button in the properties panel