Honestly I do not think that Messi and Neymar and Mbappe would have any problem if you ask me, those are great players who will definitely be doing great together, we are talking about probably the best trio at offense we have seen in recent memory, definitely amazing talents. What I really wonder is that Ramos vs Messi, this is the same two players who fought against each other for over a decade right now. That should tell you what you need to know about the current situation, that should not really be that easy to handle at all.
I am not sure about your memory but I remember some much better trio, for example, Messi, Neymar, Suarez or Mesi, Ronaldinho, Etto... it's some recent history. Or Ronaldo, Zidan Figo...
we will probably never know how good they are because they play a very uncompetitive league. Maybe they can win the Champions League
I remember Tottenham in the final game 
but it will definitely help them if they have easier games in the France league.
I would definitely expect them to fight, think about how many times Ramos tackled Messi very hard and nearly injured him, and Messi clearly will not like him because of that, and think about how many times Messi passed Ramos and scored, hence why Ramos will hate him. It is obvious that it is not about ego but the history they have.
They play for millions $$, I don't believe he will complicate unnecessarily. plus they're all old enough already and they only do this for money.