3 separate companies when offer 3 different services, it’s good to have 3 separate websites as 1 website for 3 different companies can be confusing from the user end and you will witness people bouncing from the website without completing the Goal.
Another thing that I really wanted to clear is that no paid traffic helps in improving the organic SERPs, this is the myth that most website owners have in mind which have nothing to do with reality.
Having 1 separate website for each service will help webmaster to easily work on it and get it rank well from the desired key phrases at the same time it will be easy for visitors to understand what service your company offer and use it when required.
Hope this helps!