Economic power is the ability to buy and sell, the ability to make contracts. It's power exerted in the marketplace, in the context of trade. Political power is the power of government and power obtained through the political process, such as by getting laws passed favoring the your purposes. Both economic power and political power are instances of control over others.
Social power generally is the ability to induce other people to do things you want. Each exercises control in a radically distinct way. Political power is rooted in an ability to harm others. By contrast economic power is rooted in the ability to offer others benefits.