Croatia is an official Candidate for EU membership, and as such is (I believe) both expected by EU and internally motivated to expect and accept CE Mark and EU regulations and procedures generally.
I have no specific information on registration, since I'm external to EU and therefore we use an Authorized Representative to handle all such matters for us.
Both Bosnia-Herzogovina and Serbia are applicants for Candidacy for EU membership, and as such are similarly motivated to expect and accept CE Mark and EU regulations and procedures.
I'd guess in the latter two countries though that there might be less assurance that a prior local system might not be applied, compared to my expectations for the former country because of their different statuses.