How to export 3-band raster geotif and make background transparent

Hi, I'm a nuub and need to export raster geotifs in 8-bit, 3-band format but keep the background transparent. When I try using the 'make background pixels transparent' in the export menu, it adds the 4-th band. My client can't use the 4th band. Is there another way to do this?
Susan
Susan