Then you might want to make sure, that signal C is never green, when signal A is red. The Trigger Multiple Signals rule will set one or more signals to red when one or more triggers are touched by trains.
But that would defeat the purpose of signal C. Signal C is there to allow full-speed running for the maximum distance before a train has to slow for a red signal at A. If C is red when A is red the trains will run at half speed all the way from the preceding block.