Criteria - The criteria for entry is that you are female and over-50. (Criteria refers to a range of attributes or requirements. "Criterion" refers to just one.) Circumstance - I paid for the meal and in such a circumstance the least you could do is thank me. Scenario - Imagine the scenario: you are walking down the beach on a moonlit night when suddenly you hear a cry coming from the water. All the words mean different things as you can see, though criteria, or rather criterion, and circumstance can be used in a similar way. Criteria to me would mean a requirement that is more fixed whereas circumstance can perhaps be more flexible. Circumstances rather than the singular circumstance is much more commonly used. Scenario basically describes a scene or chain of events. Hope that helps, Ruby.

Criteria are a set of conditions that must be met before something can happen. 'The university entrance criteria include english speaking competence, and a school qualification'... 'the criterion for a pay rise is that you have exceeded the job requirements'. Scenario is a (usually imagined) set of conditions. It is usually posed to query people's reactions to that scenario. 'Imagine the scenario where there is an earthquake on a snowy night...'. The training scenario involved a fire in a petrol station. Circumstances is a set of conditions that come together, often unexpectedly. It is similar to a scenario, but the difference is that it is usually used to describe a real set of conditions. 'The circumstances that led to the accident were the unusually hot weather, combined with high winds'. [someone having mentioned a couple of things that went wrong] - in those circumstances, I would not pay for the meal.

The three words are different in meanings, and I would not use any of them interchanegably. You can check the definitions in the dictionary.