Hi cub682,
Adding a topper can certainly help with a mattress that is too firm and isolate you more from the support core but I would be a little cautious about the softness and thickness when you already have a relatively soft comfort layer to make sure that you don’t compromise your alignment.
There are some topper guidelines in post #8 here that may be helpful.
Post #4 here includes some of the better sources for toppers I’m aware of including Dreamfoam which carries 3" latex toppers as well.
Phoenix