Hi Darell,
I think the best way is to capture 3 separate AVI's of the Red, Green and Blue channels. This will eliminate the possibility of accidently adding the wrong colour channels to each other.
Another way is to do what you have been doing and capture just one AVI and record what frame number you have paused on. You can then refer back to that frame knowing exactly where you have paused and separate the AVI into three parts.
I think capturing 3 separate AVIs would be much easier saving time and effort cutting it back into 3 parts.