Hi andy123,
I agree with sleeping that adding a 3" soft latex topper to a mattress that already has thicker layers of softer material in the comfort layers would be quite a big change. You can see the topper guidelines I would use in post #2 here and the posts it links to and an extra 3" of soft material would be in the range of “a fair bit to a lot” in terms of extra pressure relief and could be quite risky in terms of alignment over a mattress that already has a fair bit of lower quality softer foam in the comfort layers that will soften more rapidly over time.
The layering of the Ben Hill Plush Pillowtop is as follows …
1" PurFoam Plush
1" PurFoam Luxury Firm
1.25 oz. Celestra
Comfort - Padding Layers
2" PurFoam Plush
1/2" PurFoam Firm
1/2" PurFoam Luxury Firm
1/2" Visco Memory Foam
Correct Back Support System
Coil System: Alternating Smart Reponse Pocketed Coil
1/2" Energy Foam
Coil Gauge: 15
360 Degree True 3" Encased Edge Support
As you can see there is 5 1/2" of lower quality polyfoam in the upper layers of this mattress which would generally be considered quite soft overall and because the polyfoam is lower density/quality it will also be subject to more rapid softening over time.
I would be very cautious adding that much more soft layers to any mattress that already has comfort layers (including the pillowtop) that are that thick and soft.
I would be aware that if a latex topper is listed specifically as 19 ILD then it is probably blended Talalay which is a high quality and very durable material but not 100% natural. One of the softness levels for blended Talalay made by Latex international is 19 ILD and it typically has an ILD variation of about +/- 2 ILD across the surface. If it’s 100% natural Talalay or Dunlop then the firmness levels across the surface would have a wider variance than blended Talalay. If the latex comes from Latex International and is 100% natural then they aren’t rated in specific ILD’s and their N1 could be anywhere from 14 ILD to 19 ILD and their N2 could be anywhere in the range from 20 ILD to about 25 ILD.
Some of the better sources for latex toppers I’m aware of are listed in post #4 here.
A forum search on Habitat (you can just click this) will bring up more information about them.
Phoenix