Does anyone know off-hand the filter thread size of the hc-3?....and how much of a jump in an adaptor ring is available or feasible? I've got a 35mm adaptor that I think has a 68mm size....

The HC3 is 30mm. All subsequent ones are 37mm.

You'd need several step up rings, which I would think would add significant vignetting.

This company has a lot of different sizes. B & H also has a good selection.
Adapter Rings (http://www.camerafilters.com/pages/rings4.aspx)

Probably can be done with one ring, and since the lens is larger, not a lot of vignette risk...
HC3 is 30mm threads.

Based on the website above and B&H, looks like one would need at least three. The largest step up from 30mm I found was to 37mm, and you could then go from 37-58, and from there to 62 or 68.

However, if someone does find a 30-68mm single ring, that'd be good to know.

I've got a 37mm 2x tele and I use only one step-up ring on my HC3, and I get huge vignetting through at least half the zoom range. But a 68mm rear thread is huge and it might not be a problem if you can step up with one ring. Three, I think you're asking for a problem.