European football’s governing body on Friday promised sanctions and disciplinary proceedings after the rebel trio, having seen the other nine clubs back out.
Those nine have also signed a "Club Commitment Declaration" including a series of reintegration steps.
"The founding clubs have suffered, and continue to suffer, unacceptable third-party pressures, threats, and offenses to abandon the project," Real Madrid, Barcelona and Juventus said in a joint statement.
"....and therefore desist from their right and duty to provide solutions to the football ecosystem via concrete proposals and constructive dialogue.
"This is intolerable under the rule of law and Tribunals have already ruled in favour of the Super League proposal, ordering FIFA and UEFA to, either directly or through their affiliated bodies, refrain from taking any action which may hinder this initiative in any way while court proceedings are pending."
The nine clubs who pulled out of the Super League and are back in the UEFA fold have agreed to "take all steps within their power" to end their involvement in the breakaway league.
Additional reporting by Reuters.