The Bruins’ greatest strength during the 2011 playoffs — aside from the fantastic goaltending of Tim Thomas — was depth. Throughout the B’s remarkable run to the Stanley Cup, they got key contributions from every guy in their lineup. All four lines played well and on a given night, any of the four could be the difference-maker. Most of this season, that hasn’t really existed. The Patrice Bergeron-Brad Marchand-Tyler Seguin trio has more or less carried the offense; the David Krejci-Milan Lucic-Nathan Horton line has been OK, but can be considerably better and more consistent. But the usually reliable third line, which got so many clutch plays from Chris Kelly, Rich Peverley or Seguin in the Cup run, has not had a great impact. That changed, maybe, with Saturday’s 3-0 win over the Philadelphia Flyers, in which Kelly had a goal — set up by Peverley and new linemate Jordan Caron — and the line generally played well at both ends. “It was nice to leave the rink with a bit of a smile on our faces and feel good about how we played,” said Kelly, whose goal was just his second of the season. “I thought we played well. We had some pretty good offensive time and some pretty good chances. It was nice to score, but it was (only) one. There are still lots of games to be played. But it was something our line can build on.”