According to Chuck's timeline, the 3mm was introduced in 1952.

1951
A forged dropout with integral Gran Sport derailleur hanger is introduced that measures 6mm thick with 4mm adjusting screw and derailleur stop at 4 o'clock; easily identified by bulge formed around adjusting screw. Later the same year a new forged dropout is introduced that measures 7mm thick with 4mm adjusting screw and derailleur stop at 7 o'clock (no Sport derailleur spring hole).

1952
A new forged dropout is introduced that measures 7mm thick with 3mm adjusting screw, a derailleur stop at 7 o'clock and a spring hole for mounting the Sport derailleur.