Anything selected on an image can instantly become an alpha channel quickly and simply by going to the channels pallet and clicking the “Save selection as channel” button at the bottom of the pallet (dark rectangle with white circle in it) and then using this mask to create a grayscale alpha image for a 3D program.

The quality of the alpha channel is dependent on how well you make your selections. By far the most concise way to make a selection is to use the pen tool and painstakingly outline the shape and if need be editing those vector points to refine the selection further but this, by far, is not the fastest way. There are many alternatives to the image selection process many less precise. You need to explore these alternative avenues and use those that work best for you.

Your prior question:

How do I create an alpha image to use in a 3D software app? Do I just use the magic wand and select everything but the house and make it black. The go back and use the magic wand to select the house and make it white. Then save image (tiff for example) and then use it in 3D app?

Where you asking: “how do I create an alpha channel” or “how do I select items in order to create an alpha channel”?