I can't think of any more impressive than the Maldini line, but there are a few fairly famous second and third generation footballers, or other footballing families. Paolo's other son, Christian, was part of AC Milan's youth set up for a number of years, although never made a senior appearance, but is still a footballer.
Not three generations but Gonzalo Higuain, his dad Jorge, and his brother Federico all have senior appearances for River Plate.
Messi's grandad also played for Argentina.
Both Juan Ramon and Juan Sebastian Veron, father and son, played for Estudiantes.
Mauro Zarate of fleeting Premier League fame and three of his brothers are all professional footballers, which is a different kind of impressive I suppose, with four kids from the same family all making careers in the game.
Mkhitaryan's dad, Hamlet, played for the Armenian national team.
Eden Hazard's mum and dad were both footballers, as are his brothers, Thorgan and Kylian.
On the topic of brothers, another fleeting Premier League player, Wilson Palacios, is one of five who are all footballers.
Eidur Gudjohnsen as been mentioned as a second generation footballer, and his son is part of Iceland's U21 set up currently, so could have a third generation there.
Chicharito is a third generation footballer, but I think most were aware of that. His grandad on his mother's side and his dad have also represented Mexico.
Diego Forlan is also a third generation footballer in the same way, I think.