Steen broke down the left side as part of a 3-on-1 rush. He kept the puck and let go a drive from just inside the circle that trickled through Crawford.
The Blues (3-0) matched their best start to a season, also done in the 1969-70 and 1993-94 campaigns. Jaroslav Halak made 26 saves in his third straight win. Vladimir Tarasenko and David Backes also scored for St. Louis.
FLAMES 3, CANADIENS 2
MacDonald made his third consecutive start.
P.K. Subban had a goal and an assist for the Canadiens (1-2), who hadn't played since beating Philadelphia on Saturday. Lars Ellar recorded his fourth goal and sixth point of the season. Carey Price made 22 saves.
Despite being outshot 12-4 in the first period, the Flames took a 2-0 lead.