Lee Cattermole and Andrea Dossena were both sent off against Hull City in a game Sunderland lost 1-0, while Wes Brown was dismissed as Poyet's men suffered a 2-0 defeat to Stoke City last weekend.
Brown's red card has since been overturned by the Football Association's disciplinary panel, but Poyet is concerned his squad are setting a worrying trend.
"I can assure you they (Cattermole and Dossena) know what they've done. That's very important," the Uruguayan told the Shields Gazette.
"They know there is a limit, and they need to be careful. I hope it never happens again, but we'll see.
"We all make mistakes, and there is no explanation why you do it. It happens in a split second. But if you do it every week, you've got a problem."
The Sunderland boss had specific words of caution for Cattermole, whose red card against Hull was his sixth during his time at the Stadium of Light.
"When someone does something really, really stupid and it's repetitive, he doesn't have a chance with me," Poyet added.
"They need to be professional."
