I don't understand your question at all, but I think that answer on it you can get in built-in help of TC under Calling Procedures Declared in Another Unit topic. There you can see all what I know about USEUNIT specifier.
If you want to use any method of UNIT_C in UNIT_A without having to call USEUNIT, just use the Runner.CallMethod function.
I also want UNIT_A to use UNIT_C, but preferably without having to call 'useunit' for UNIT_C in UNIT_A.
<font size="2" face="Verdana, Arial, Helvetica">Only JScript and C++Script languages have this ability.
You can use the Runner.CallMethod method to call routines from other units. Here are the examples: