They are also British colonies. Why are America, Canada and Australia much more prosperous than British colonies in Africa?

Because the United States, Canada and Australia are the so-called new world, an unexplored no-man's land (there are Indians in the United States, but they are all about to be killed). These places are rich in resources, which enabled the development of the European technological industrial revolution brought about by European immigrants. . Africa, on the other hand, is not suitable for immigration, and British immigration to Africa is not very successful. France is much better for African immigrants. The local black population in Africa is already quite large, and there is less room for Europeans to immigrate. The climate and living conditions in Africa are also poor. Europe's development of Africa is basically only mineral trade and slave trade.