First of all, not only should your Exchange server not be on a PDC (which do not exist in W2k network), but it should not be on a domain controller at all. The exchange server in W2k network should be a member server. There re many reasons why you do not want Exchange on a DC. i.e. increased workload on the DC from mail services, DCs have FSMO roles that may conflict with the role of an Exchange server.
As for the rest of your question, you have not provided enough information to give a credible answer. Every network should have 2 domain controllers (W2k or NT domains). If you have 3 servers, and 2 of them are DCs, and the 3rd is an Exchange server, what is left....?