Short answer is the group of Knights may collectively throw together 3 merlin to cancel a special black card as a free action before it goes into play. Once it is decided to lay it on the board it cannot be cancelled by the 3 Merlin at a later time.
The Knight may continue with his heroic action normally.
Thank you so much for the quick reply!There is nothing on the rule book saying this is a free action so i was really unsure about this. Thanks for the clarification.