In claiming his second stage win, Sagan, who also won Stage 1 Sunday, finished the 197-kilometer trek from Orchies to Boulogne-sur-Mer in 4 hours, 42 minutes and 58 seconds. That was a second faster than a large group that included Cancellara.
Cancellara won the prologue Saturday and is still 7 seconds ahead of Britain's Bradley Wiggins and Frenchman Sylvain Chavanel, who led for most of the final 5 kilometers Tuesday before being caught. Defending champion Cadel Evans of Australia finished sixth Tuesday and is 17 seconds off the lead pace.
Stage 4 will cover 214.5 kilometers from Abbeville to Rouen Wednesday, with riders rolling along the English Channel coast for most of the way before heading inland. It features four Category 4 climbs.