Hi SideSleeper,
I’m sorry to hear that your Simmons Natural Care mattress didn’t work out for you and if there are visible impressions that are 3" deep then there is no doubt that at least one or more of the layers in the mattress were defective.
Unfortunately, and like many of the members here that have been in a similar situation, you are in a somewhat difficult position where you need to exchange a mattress at a store where there may not be any particularly good quality/value options available to you.
There are some suggestions and ideas in post #2 here about the two main strategies that you can use that can help you make the best of a difficult situation. There is also more in post #13 here about the most important parts of the “value” of a mattress purchase or exchange that may be helpful as well.
Once you have a better sense of how to approach your exchange if you have any specific questions that I can help with then I’m certainly happy to do so to the best of my ability.
Phoenix