Ron,
first of all, thank you for your information. I've entered the site you mentioned and found out that HR3C is the kind of alloy that here in Brazil we know as "25 - 20 alloy type", i.e., that containing 25% chrome and 20% nickel, plus small quantities of other metals.
Although possible and feasible, those alloys are not easy to weld due to their high chrome and nickel content.
Also, those alloys use to be called "proprietary alloys", because they're protected by patents that belong to their manufacturers.
Among those alloys, I recall the followings: Carpenter 20, Hastelloy C, Jessop 700, all of them made in the U.S.A; Uddeholm 904, 2RK65 and Avesta SLX, made in Sweden; Uranus B6, made in France.
My opinion is to get in touch with the manufacturers, either by e-mail, fax or telephone (telephone is better because it's a personal contact) and ask for their advice.
Secondly, thank you, Ron, for calling me Doctor, which I'm not. I'm just a BS in Chemical Engineering.
Giovanni S. Crisi