Efficacy is the capacity for producing a desired result or effect in an ideal setting. Does the agent or intervention “work” under ideal, “laboratory” conditions? For example, a medicine could be ready to improve a patient’s symptoms in a perfect environment, where they’re closely monitored in order that they stick perfectly to the prescription. In this case, the drug has demonstrated efficacy.
Effectiveness is the extent to which an intervention produces the desired effect in real-world or everyday conditions. While an intervention may have high efficacy in a perfectly controlled setting in experiements, it may not work as well to the same extent within the real world to a broader population.
Efficiency is comparing a solution’s input to its output, taking into the account the economic aspect. For example, two drugs might be equally effective. However, if one of them is far more costly than the other one, that drug won’t be considered as efficient.