Hi SleepDeprived,
It may be worth trying to replace the top talalay layer with your 3" memory foam topper (if you still have it and if it fits into the cover) or to try a 2" piece of softer polyfoam (which is also a fast response material like latex and would be relatively inexpensive) to see if the “jiggly” sensation you are experiencing is reduced or eliminated in which case you can assume it comes from the top Talalay latex layer. If the source of this sensation is the Talalay latex then a thicker topper will dampen it more but it would also be more risky in terms of alignment if the topper was also in the soft range (you would be adding an additional 3" of softer material to the 2" that are already there which would increase the likelihood of sleeping out of alignment). A thinner topper will dampen the sensation less but will also be less risky in terms of alignment. A 2" memory foam topper would be somewhat “in between” so it would provide more dampening than a 1" memory foam topper and would be less “risky” than a 3" memory foam topper but it may also end up being not enough dampening and too much thickness/softness.
Some of the better sources for polyfoam and memory foam I’m aware of are in the component post here.
Talalay is naturally a more flexible, resilient, and “floppy” or “jiggly” material which is the reason that some people like it so much but is also the reason that some people don’t.
Phoenix