If you have the individual negatives, you can invert them and use the Combine Channels command to merge them into an RGB document. You will need to apply curves to account for the negative transfer characteristics and make them look natural again.
Okay, you probably want to know how.
Create a new, blank document the size of your scans. This will be your composite.
Repeat for the other two colors.
Great suggestion. Will try it and let you know. Thanks