Well, surprisingly you looked at the same mattress but under different names at those three stores. This practice has been going on for decades, referred to as Mattress Name Game. It is a tactic by mattress retailers to keep customers from comparison-shopping and to keep their profit margins inflated. Retailers and manufacturers alike purposefully provide limited product information on labels, price tags or online to keep you from doing your due diligence.
All mattresses of one brand of a particular manufacturer are made at the same factory but they will be assigned different name depending to which retail store they will be shipped to. Typically, there could be differences like material color or different pattern on top but the construction, major features, and overall feel remain identical. Retailers can claim that they technically have a unique model with a unique name after applying these minor changes. Almost every major player in the mattress industry plays the name game. So the same mattress can have numerous names and different prices, in-store and online, which makes it almost impossible to compare.