Merging different bands to RGB in ER Mapper

I have the following image data available to create a RGB image:

blue.ers

blue.tif
red.tif
green.tif

How can I combine the three tiffs and save it as an rgb tiff file using
ER mapper?

ER Mapper shows a tutorial on how to add/remove bands using an RGB
image and the algorithm wizard, but doesn't explain how one can merge
three bands into one.

Any help is greatly appreciated!