Hi ddcfri,
Assuming that the support layer is still in good condition which is quite likely … I don’t see any reason why not although of course it may take some care and effort to remove the memory foam if it’s glued to the polyfoam support core.
There are too many unknowns, variables, and personal preferences involved for me to recommend anything specific for someone else in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). If I was in your shoes however I would probably start with a single 3" layer rather than take on the complexity and possible trial and error of multiple thinner layers. If the 3" memory foam layer you choose doesn’t work as well as you hope when you sleep on it then you can always add a topper on top of the mattress to provide additional pressure relief.
Different types of memory foam can be very different from each other even if they are the same density (see post #9 here and post #8 here) but in very general terms 5 lb memory foam will tend to be firmer than 4 lb memory foam. Also as a very general guideline … gel memory foams will also tend to be faster responding and less temperature sensitive than non gel memory foam.
When you can’t test a specific combination of materials in person then you will be dependent on a conversation with each of the suppliers you are considering and their ability to describe the properties of the memory foam they sell relative to other types of memory foam on the market so you can choose the one that you think you would prefer.
3" of memory foam may not be thick enough for many side sleepers (a mattress with a 3" memory foam layer over a firmer support core will tend to be on the firmer side) … so I would start with the 3" memory foam layer and then use your experience on the mattress as a reference point to help you decide whether you need an additional memory foam topper for extra pressure relief.
Since you need a 3" layer to fit your cover which may be too firm anyway … I would probably be tempted to use a 3" 5 lb layer first with an eye to adding a softer 4 lb topper (probably around 2") which would turn the 5 lb comfort layer into a transition layer and may provide better alignment and support than if you had 5" of 4 lb memory foam.
Some of the better online sources for memory foam toppers or layers are listed in the component post here.
Phoenix