It'll be pretty negligible.
Kennedy's numbers are more or less within the margin of error. He is more popular with Republicans than Democrats, hence why Trump camp wants him out of the race.
If likely Kennedy voters have to vote Trump or Harris then yeah the split will likely favour Trump but in tiny numbers.
West is more of a problem for Harris as he is popular with young and Black voters, again not in massive numbers but not insignificant where he can get on the ballot - this is why Trump campaign legal team have been helping him.
Kennedy was thought to be attractive to 2020 Biden voters who didn't want to vote Biden this election and they've come back to the Democrats already according to some polls along with some independents.
All in all there's not a lot of voters to share around who are currently Kennedy voters.
NBC have a piece on this very subject:
Surveys show that most of Trump's third-party defectors have been picking Robert F. Kennedy Jr. on test ballots with more choices.
www.nbcnews.com