Yes, the Le Mans Circuit de la Sarthe is too long for Grand prix 3.
I understand that the one in the new "Le Mans" game is rather good.
The addon ones for Sports Car GT (1986, 1989 and 1999 versions) and Grand Prix Legends (1967 version) are all excellent. The Sports Car GT conversion of the 1999 version to F1 20002 has no AI or night versions as I understand.