Hi pickamattress,
I’m not sure what you mean by a “foundation” and a 3" topper. Do you mean putting a 3" topper directly on a mattress foundation that is meant to go under a mattress so that you would be sleeping on only 3" of latex? If this is what you mean then I think that a 3" mattress on a foundation would be too thin and uncomfortable for most people.
If that’s not what you meant then could you clarify what you are considering a little more specifically?
Your choices will basically boil down to which local mattresses you have tested are your “finalists” based on your testing for PPP and the other parts of your personal value equation are most important to you and which of the online options you are considering are your “finalists” based on your more detailed conversations with them (instead of personal testing) and also the other parts of your personal value equation that are important to you including the risks involved in each one and their return or exchange policies (to reduce the risk) just in case what you choose doesn’t turn out to be ideal for you.
Once you are down to your finalists and they are all choices between “good and good” then you can make a final choice between them.
Phoenix