The only way i can picture doing this would be to have an Automation which would create individual child records when the Multi select record is saved. it would need to create a child for every one of the selections, and then based on a relationship to the parent multi select field total up the # of children, and then look that up down to the children so each could calculate what weighting to receive, say 1/3 or 1/3rd.
You would need to start off the automation by deleting all its children for that multi select Parent in case the user edits the multi select, ie delete the children, if any, and then re-create the children.
