Using ArcGIS 9.3, I would like to make a new, single-band raster file that includes only one value: a summary of the three existing bands. The composite band is currently a composite .tiff file. Something like Band1 + band2 + band3 = newValue , for each pixel in the raster. The new raster would contain only one output band, with the values for that band = newValue.
You can do this through Spatial Analyst.
Adding separate bands is very simple. When you are adding layer, go to your tiff and instead of adding, double-click on it. You will enter into its bands and now you can add these bands separately.
So, from this
you will enter into this
Then you can use Raster calculator to do the sum. Open it from the Spatial Analyst toolbar.
And just write your equation for the 3 bands.
Then you will get a temporary raster which you can save on your computer.
Hope this is what you need.