What a good question! I will try to answer it as simply as possible.
“Eficaz” means “effective” as in “es una política muy eficaz” (it’s a very effective policy). Effective can also be translated as “efectivo” although it’s a lot more common to hear “efectivo” used to mean “cash” (as opposed to credit cards or cheques). As for “eficiente”, this usually refers to things that save money, or have a low consumption of petrol or electricity.
Well, I’m glad I cleared that one up for you – with a bit of luck this explanation has been “efectivo”, “eficaz” and “eficiente” all at the same time!