Hi IrishST.
Welcome to our Mattress Forum! 
[quote]The specifications of each of the foam layers used in the hybrid mattress are as follows:
1" (1.8lbs) Luxury quilted cooling gel foam that helps keep you perfectly cool
1" 3 lb gel memory foam that provides superior relief for pressure points
1" (1.8lbs) High Density foam that provides support[/quote]
While generally speaking 1" of 3lbs memory foam would be a lower quality/density material I’d keep in mind that for normal BMI sleepers one inch or so would be within the acceptable range without having a significant impact on the overall support/comfort and mattress performance over time. There should be no more than “about an inch or so” of lower quality/density materials in the comfort and quilting layers combined (around an inch or less would not have a significant effect on the durability or longevity of the mattress). Once you are at about 2" or more of lower quality materials, they are likely to be the weak link of the mattress. I would, however, place a caution for higher BMI ranges as this mattress has a total of 3" of foams that have densities lower than what I generally recommend for higher BMI individuals. 1" at 3lbs in combination with the 1" of 1.8" lbs of cooling gel on top and the 1" of 1.8lbs HD would be lower quality/density" material (relative to higher BMI only)
As a brief summary, higher BMI ranges will need more durable materials and components in a mattress and in a BMI range of 30 or higher I would include any 1.8 lb polyfoam or 4 lb memory foam as a “lower quality/density” material (relative to a higher BMI only) and minimize their use to a total of “about an inch or so or less” in the mattress. For polyurethane foam, if your mattress is one sided then I would look for 2.0 lb per cubic foot density or higher. If the mattress is two-sided then I would use a minimum density of 1.8 lbs per cubic foot or higher. For memory foam (or gel memory foam), if your mattress is one sided then I would make sure that any memory foam is at least 5 lb per cubic foot. If the mattress is two-sided then I would use a minimum density of 4 lbs per cubic foot.
A high BMI presents special challenges and generally requires firmer materials (in the support layers especially). There are also a number of forum posts and threads about mattresses and higher weights and a forum title search on heavy (you can just click this) will bring up many of them. Some examples include post #2 here and post #2 here and this thread and Post #3 here has more information and suggestions about heavier weights.
As Jewels mentioned when assessing any product, also be sure that you find out information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
The first place to start your research is the mattress shopping tutorial here(there is a condensed version of it at the end of that post) which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones. Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well he will sleep), durability (how long he will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
I hope this gives you a good idea of how to start looking for better options and what to look for in your search for a new mattress.
Phonenix