What covers the center of canada's 3 northern territories?

The central parts of the three northern territories are covered by the tundra.

Canada, in general can be easily divided by the biomes that cover its territory, by simply saying that the southern half of the country is covered by the taiga, and the northern half is covered by the tundra.

The three northern territories are Yukon Territory, Northwest Territories, and Nunavut. All of those are in close proximity to the Arctic, and the climate there is very cold for most of the year, with snow and ice dominating the landscape, and having only a very short fresh, relatively cold summer.