Canada is larger by total area than Brazil or Australia. Wikipedia's table lists Canada at about 9,984,670 square kilometers, compared with Brazil at 8,510,346 and Australia at 7,741,220 square kilometers.
Canada's lead comes largely from its immense Arctic territory and enormous inland-water area. Its landscape stretches from the Atlantic and Pacific coasts to the Arctic Ocean and includes thousands of lakes. Brazil is the largest country in South America and has a greater land area than Canada, but its inland-water total is much smaller. Australia is the world's smallest continent and largest country in Oceania, yet its arid interior limits its total size relative to the other two.
A frequent mix-up is the claim that Brazil is larger than Canada. That can happen when comparisons use land area only, different boundary conventions, or exclude some Canadian inland waters. By total area in the referenced table, however, Canada is clearly first among the three.