One thing is clear: Africa did not benefit most from the slave trade. Some African nations became rich and powerful (in their areas) but many others were devastated by the loss of human resources.
Of the other two, I would argue that Europe benefitted most. America did gain a lot -- it got the benefit of the slaves' labor. However, the US had to fight a bloody civil war because of slavery and all American countries that had significant amounts of slavery continue to suffer from social problems that are at least somewhat related to the existence of slavery.
Europe, by contrast, got less benefit. Europeans did, however, profit from slave trading and from the slave economies of their colonies. In addition, they escaped to a large degree the social problems that came from slavery.
i would concure with the answer below about afica, but would tend to differ with the number 1.
yes alot were killed in the civil war but the slave trade kept the south alove with labor for a long time producing alot fo good to sell and trade, which made americas economy boom, which i wouls say is why the benefited more