Hi SeattleJen,
It’s hard for me to imagine a situation were a mattress needed two 3" toppers although your higher weight may be an exception because your experience on a mattress or “sleeping system” will be outside the norm.
I think you may be making changes too quickly. I would make sure that you sleep on a mattress for at least a couple of weeks before adding any toppers at all. While a specific symptom may not appear until you have spent some time on a mattress (particularly with much higher weights where discomfort is more likely regardless of the mattress you purchase) … if you do some careful and “objective” testing in all your sleeping positions then you will usually have some more subtle “cues” that can come closer to predicting your longer term sleeping experience on a mattress. Good testing will usually result in a mattress that is “close enough” to the level of comfort and support you need to allow you to sleep on it for a couple of weeks. Of course your actual sleeping experience now is also your testing so I would make sure that you sleep on each combination for as long as possible before taking the next step.
Once you have slept on a mattress for long enough to identify any changes that are necessary … I would make one change (or add one topper) at a time based on your experience and “symptoms” on the previous mattress or combination you tried. No matter how closely your testing or sleeping experience is able to predict your long term experience … you are the only one who can feel what you feel on a mattress and your own testing and experience will always be more accurate than anything else and everyone else’s thoughts and ideas are only “guidance” and secondary to your own actual experience which may be different from what anyone expects. The topper guidelines in post #2 here would help with each change although I would add some thickness to the guidelines to compensate for your greater weight. I would then sleep on any new combination for at least a week or so before changing or adding anything else.
I would also keep in mind that when you are dealing with unusual health or physiological situations such as weight issues that there may be no mattress that is a “perfect” solution by itself and that more customized approaches may be necessary to get to the “best possible” sleeping system that a mattress can provide. Post #11 here has some ideas that may also be helpful about more customized zoning.
I would also make sure you are dealing with the most knowledgeable person at the retailer where you purchased the mattress (or ask them about who you can talk with that may have more knowledge or experience with people that are in similar circumstances to you).
It may also be helpful if you could list all the specifics of what you are sleeping on in the forum (the type, thickness, and firmness level of each layer and component in your mattress and each topper) which may also provide some clues about what may be your “next best” step.
Phoenix