i don't believe they have ever used anything but regular jerseys in the playoffs. Not sure if it's a rule. Just an observation.
Edit:
http://en.m.wikipedia.org/wiki/Third_jersey
Teams are also allowed to wear their third jerseys as often as desired in the Stanley Cup Playoffs; however, they may only wear two uniforms during the playoffs. Examples of teams who wore third jerseys in the playoffs include the Dallas Stars (during the 1999 Stanley Cup Finals), the San Jose Sharks, the Anaheim Ducks, the Minnesota Wild, the Washington Capitals, and the Toronto Maple Leafs.