Catacombs of the Talion
The first mask is located inside a cage in the first large room with the pool and Snow Leopards. Lara first needs to get down from her initial position and then climb a ladder in order to get to a switch that opens the cage. The Mask opens a metal door to the Level's main area.
The second mask is located in a small pool by a large glacier, near the frozen pond, in the area around the large fire urn. It opens a door to a yeti-infested room back in the main area.
The only mask in Ice Palace is found on a small block in the area after the level's opening vicinity. it is guarded by four white tigers. It opens a door to a switch which lowers the fire urn seen in the previous level. In fact you don't need the mask to get to this switch at the location of the fixture, but getting to that area requires picking up the mask, as it opens a nearby door which leads there.