Gold Coast, St George Illawarra and Newcastle are facing fines totalling $350,000 after being issued with breach notices for failing to follow the NRL's concussion rules during round three.
The Dragons were cited for an incident involving Josh Dugan and the Knights sanctioned after Brendan Elliot sustained a head injury.
"These are, by far, the heaviest fines ever proposed by the game for concussion breaches," NRL chief executive Todd Greenberg said in a statement.
"That is how seriously we take it.
"The clubs involved have the opportunity to respond to the Breach Notices, and we will consider those responses, but our message is clear ... we are not going to allow player safety to be put at risk through breaches of the concussion rules."
Greenberg said the NRL was committed to showing a duty of care to the players.
"In the majority of cases we see strong compliance with the League's concussion rules but it appears that this did not happen at the weekend in some matches and we cannot stand by and allow player safety to be put at risk," he said.
"Where we believe the rules have been breached we will take action - and we would hope that these breach notices will serve as a warning to all clubs."
The three clubs have five business days to respond to the breach notices.