I have an 8 year old acrylic shell with fiberglass backing.
The shell has developed three small (size of a quarter) blisters.
Any tips on compounds to use, and techniques to use????

I would guess that the blisters are in the footwell area, near the edges. This is somewhat common, with no easy solution. The cause is from the acrylic separating from the fiberglass, and is usually caused by moisture getting between the two layers, heat damage, or just a weak bond that came loose.

Although it doesn't leak, it will get worse. Eventually the acrylic can crack and flake away from this area, leaving just the fiberglass shell.

The blisters come from air bubbles trapped in the sandwich between the acrylic and the fiberglass during production.
Time, heat, and microbiology combine to form the blisters.
Restoration involves bubble removal with a dremel tool, (acrylic layer only), and fill with a compatable epoxy. Sand smooth. The manufactur may be able to provide a repair kit to address the colour match issue, but I was not concerned because the blisters were under the water, and very small. Next thing - add water, heat, Jill, and enjoy!