Hi Pspa123,
From your description it sounds to me like the topper may be a little too firm (which can create pressure points) or a little too soft and thin (which can allow her to “go through” the softer topper and feel more of the firmer layer below it which can also create pressure points). Both the core and the topper will act together and each will have an effect on the other but a medium core would be suitable for many people (depending on body type, sleeping style, preferences, sensitivities, physiology, and other things that can affect comfort or support on a mattress) so I would probably focus on the topper unless there is an obvious reason to look elsewhere.
The first thing I would do is to talk with the manufacturer of your mattress and/or of your topper. They will often have a great deal of knowledge and experience with their specific products and are usually the best source of advice and this also give you the chance to have a more detailed conversation on the phone which is usually much more helpful than email or forum exchanges that rely on written information and “theory at a distance”.
Other than that … the topper guidelines in post #2 here may also be helpful although I would be very careful with adding too much soft latex on top in addition to the 3" you already have because of the risk of alignment issues unless you were confident that you wouldn’t be taking on the risk of exchanging a pressure issue for an alignment issue. In general “less is better” as long as it is thick enough to solve the symptoms you are looking to solve.
To make an educated guess more specific than that would require much more detailed and specific information about both her, the mattress, and the topper and possibly a more detailed phone call with the mattress manufacturer but if I had to guess she either needs a 3" topper with a lower ILD (if the topper itself is too firm) or a little more thickness in addition to what you already have (if the topper is too soft/thin to isolate her from the firmness of the support layer below it when she is on her side).
Phoenix