If you played either of them with two scrubs, Bergeron alone could make it a respectable line. The other two would have a harder time.
That being said, I think Marchand and Pastrnak form a better duo than Bergeron+Marchand/Pastrnak.
Bergeron is brilliant at gaining possession of the puck and protecting the middle of the ice. Over the years he has developed a hell of a quick catch and release.
Marchand can snipe, get into dirty areas and has amazing edge work in the offensive zone he uses to create space to pass or shoot.
Pasta just knows how to score. Crosby like (style wise not production wise of course) in his ability to score in all kinds of creative ways.
This is like choosing your favorite kid as a Bruins fan. Bergeron is probably the best, but oddly also the easiest to replace on the 63/37/88 line.