Interoperablity issue acoross machine in the same network

Question

I am working on a win form application which need to support 3-tier installation i.e
1 Machine for DB Server
1 Separate Machine for Application Server: It is actually a delphi exe
1 Separate Machine for client:WinForm C# application.
For communication between client and App server i am using interop which is working fine in standalone or 2-tier environment(Client and App Server both ton same machine) but in 3-tier environment its not working.
I cannot deploy webservice also on app server.
So can anybody help me in accessing application server through client in 3-tier environment.
Any help will be appreciable. Thanks