The Bruins haven’t lost many times this season: four regulation losses for Boston on the year to be precise. But almost all of them have involved folding like a deck chair in the third period when things get a little bumpy, and that is not the kind of Bruins hockey people are accustomed to. It happened again on Tuesday night as the Bruins blew a two-goal lead in the final seven minutes of the third period, and dropped a 3-2 game to a surging Pittsburgh Penguins team at the CONSOL Energy Center. “After the second period I told our guys we couldn’t just sit back and defend,” said Claude Julien. “In the third period it was about playing our best period, but there was some really bad decision-making by us that led to those goals. We gave that game to them.” These kinds of things happen during a regular season to the best of hockey clubs. But it’s a shock to continually see third-period meltdowns from a team that has long owned the final 20 minutes of games. They have still outscored the opposition (21-15) over the balance of third periods this year, but it sure doesn’t feel like that right about now. The Bruins alternate captain wasn’t biting on the fatigue excuse during the second night of back-to-back games. “I don’t think it was about the energy. I think it was about the way we approached that third period,” said Patrice Bergeron, who assisted on Tyler Seguin’s first period goal and dominated Sidney Crosby in the face-off circle. “I don’t know exactly how to explain it, but it’s not acceptable.”