Hop David wrote:

It seems to me our 4 gas giants would have lots of 3He. Helium is a gas
at Neptune distances. How would the lighter bodies in the Kuiper Belt
accrete a gas?

They wouldn't; it might be trapped in growing ice grains.

Alternately, 3He is produced in situ by spallation. IIUC, the trapped
helium in some meteorites is about 2% 3He (the rest 4He, much of that
from radioactive decay.) I don't know the absolute concentration of
these isotopes, unfortunately.

Paul