Hi Mamie,
It can be somewhat difficult to choose the best topper for a mattress because how it feels and performs will be a combination of your topper and the upper layers of the mattress that are below it in your mattress. A mattress can be very firm but still have some very soft layers in the upper layers (the firmness can be the result of “going through” the top layers and feeling the firmness of the layers below them).
There are some topper guidelines here which should be helpful in helping you make a topper choice that has the best odds of success. The post also includes a link to some good sources although it’s always “safer” to make a purchase of a topper that you have tried in person. The “safest” way of all to make a choice would be to test the specific mattress / topper combination that you are considering because how a topper feels and performs will change depending on what is underneath it but this is not always possible so you then need to make an educated guess about the topper you think would be best.
As you can see from the memory foam guidelines here … I would tend to avoid 3 lb memory foam for durability reasons although in a topper when it softens and degrades faster than a higher density memory foam it can be replaced without having to replace the whole mattress so it’s less “risky” in a topper than it would be in a finished mattress.
Dunlop latex will also feel firmer than Talalay latex at the same ILD because it is a denser material that gets firmer faster than Talalay when you sink into it. You can read a bit more here about the differences between them which may help you imagine the difference in “feel” although again personal testing is the most reliable way to tell how they both feel to you. Both are great materials.
A good “average” topper choice would be 2" in a relatively soft ILD (what feels soft for you will depend on your body type, sleeping style, and on the layers below the topper because they will also be a part of what you feel with a 2" topper).
A medium firmness / density Dunlop topper may be on the firm side for many people for use as a topper … especially side sleepers or those who are lighter … but again this will depend on your personal circumstances and preferences.
Phoenix