The clubs were part of the 12 ‘founding’ members of the breakaway league, which later collapsed.
In a statement released on Tuesday, Uefa said: “Following an investigation conducted by Uefa ethics and disciplinary inspectors in connection with the so-called ‘Super League’ project, disciplinary proceedings have been opened against Real Madrid, Barcelona and Juventus for a potential violation of Uefa’s legal framework.
“Further information will be made available in due course.”
The other nine clubs involved – Arsenal, Chelsea, Liverpool, Manchester City, Manchester United, Tottenham Hotspur, AC Milan, Inter Milan and Atletico Madrid – have been punished by European football’s governing body for their participation.
They have agreed to make a combined 15m euro (£13.4m) goodwill contribution to benefit children’s and grassroots football across Europe.
They will also have 5% of Uefa competition revenues withheld for one season, starting in 2023-24, and this money will be redistributed, including in the UK.
Each has also committed to Uefa and its competitions, with the clubs facing fines of 100m euros (£86.9m) apiece if they seek to join an unauthorised competition in the future. They will be fined half that if they breach any other terms of the declaration.