To help out some research, this is actually one method of grafting called "approach grafting" where the plants are joined at the stem part way up and you slowly remove the lower portion of plant A so that it survives off plant B's root mass. In your situation you want to keep the root mass from all of the plants, so no cutting involved. I juggest shaving the stems where you want them to join, and wrapping them up until they heal together though.
if you search for approach grafting, you'll understand exactly what is happening to these fusing stems.
hope this helps, cheers