First you have to put a Flag Warp in screen B that warps you to Screen C: for example, if screen C is 1 screen to the right of screen B, the lines to add to the screen B section of the World.ini file would be
...
Flag(A)=x
FlagWarpX(A)=1
...
Where x is the Flag you chose (you can use B or C instead of A, it doesn't matter). If you use the KS Manager instead... I don't know, I've never really used it.
After doing that you should determine the correct way to set the Flag that triggers the Flag Warp, which depends on what the trigger you want to be. The only ways to set Flags are Shifts (works for any Flag) and picking up Powers (only works for Power Flags, of course); if you want the trigger for the screen change to be just entering screen B, then you should probably put a Shift in screen B, at the very entrance of it. In any case, the code to add would be ShiftFlagOn(A)=x (again, using A, B or C is the same).
If you still have trouble understanding, you should post what you're trying to do in your level (the relevant sections of the World.ini file, some screenshots or even the level itself).