I've no access to the EXE file (I assume it's a Genesys thing) but there are two ways you can try: concatenating the parameter strings together with spaces between them:
Process.Start(@"C:\Users\zzzz\Documents\SCServer_mlcmd\mlcmd.exe", String.Join(" ", rut_ors_p, rut_messagesrv_p, etl_p);
Or issue three separate Start commands:
Process.Start(@"C:\Users\zzzz\Documents\SCServer_mlcmd\mlcmd.exe", rut_ors_p);
Process.Start(@"C:\Users\zzzz\Documents\SCServer_mlcmd\mlcmd.exe", rut_messagesrv_p,);
Process.Start(@"C:\Users\zzzz\Documents\SCServer_mlcmd\mlcmd.exe", etl_p);
Which you need is up to how the app was written in the first place.