To link multiple Assembler modules where all the calls reference the external names of the called modules, you need to ensure that Automatically link module and Resolve external calls at link time are checked for the main module (the calling module) and unchecked for the called modules.
You can find both these check boxes on the Link page of the Property Pages for the module or the Assembler tab on your project properties Assembler page.
Since all the calls reference the modules' external names, the Assembler Compiler finds and locates all the called modules in your Assembler source libraries, and builds the main and called modules into one .390 file. Once this .390 load module is brought into memory by the COBOL run-time loader, all public entry points in the .390 module are callable by any other COBOL or Assembler module.