I have an ERDAS IMAGINE file that has only 1-band. When I add the file to a map on ArcGIS pro, it shows up on a grey scale. I'm hoping to somehow find out if there is a way to convert it into 3 band file without the need of a colormap. The file did not come with one included.
A 1-band image will not get displayed as a multicolor RGB, even if you make a composite of 3-bands using the same band. Unless you have separate bands that you would like to use as a Composite, you shouldn't create bands composite. Showing the same band in RGB will also display a grayscale image.
Instead, you could change the symbology of the Raster layer. Check Change the symbology of imagery—ArcGIS Pro | ArcGIS Desktop
In addition to what Jayanta Poddar is indicating, your information (single band no colormap) does not have information about color. You will have to use the unique renderer and define the colors for each unique value.
In ArcMap there is a way to use the Colormap to create a temp raster that can be exported as RGB, but I'm not sure is there is an equivalent in Pro: