Hi tlic sss,
There are several different issues involved here.
First of all its true that softer foams of any type won’t be as durable as firmer foams of the same type but this mainly applies to the surface layers because the deeper layers don’t compress as much. The force of your weight spreads out as it goes deeper into the mattress and some of it dissipates (this is called hysteresis) so since the upper layers are absorbing more of the weight and the weight is spreading out over a larger area … the deeper layers will compress less and be more durable than they would be on the top of the mattress.
When you have a firmer layer over a softer layer … it’s what I call a “dominating” layer. It has a different feel than the other way around and some people like the feeling of being on a firmer foam where they don’t sink into the foam as much but may still like some extra softness underneath for good alignment and a different “kind” of softness. The firmer layer on top won’t compress as much and will “bend” more into the layer below it. Typically this is done with thinner firm layers either on top of the comfort layers or in the quilting with the softer foam underneath. Some people (including me) even like a thin layer of slightly firmer latex over softer memory foam so you have the more resilient surface feel with “slow” softness to sink into underneath.
With 3" layers … they would typically be layered progressively (softest on top) but your own experience would be the guide I would go by. There is no right or wrong here. For example if you had a soft layer over a medium layer you would sink in a little deeper than if you had a medium layer over a soft layer. If the medium over soft is the perfect feel for you then it would be “right”.
As another example if someone liked the surface feel of a medium over medium but they wanted to sink in just a little more without giving up the surface feel they liked … then a medium over soft could work well for them. As a more extreme example of what I mean by “bending” into the layer below it … if you imagine a 1" layer of very firm foam that is flexible but too firm to compress easily and this was on top of a 6" layer of softer foam … the 1" layer wouldn’t compress much under your weight and would feel firm but it would “bend” into the softer layer below it. It would have a “different kind” of softness with a firmer surface feel. The same thing happens in a less extreme way with any dominating layer.
Thanks too for sharing your experiences at SleepEZ. It’s always great to hear about some “in person” experiences with some of the manufacturers who mostly sell online. 
Thicker layers do have a different feel to them that some people will notice and some won’t (depending on their sensitivity, body type, and their sleeping positions). Once again there isn’t any right or wrong. While for most people 8 or 9 inches of latex is easily enough … for some the extra thickness has a better and more “adaptive” feel. There’s more about the effects of thicker layers and mattresses in post #14 here.
I’m looking forward to hearing about what you end up with.
Phoenix