After many phone calls to plumbing supply shops and some internet searching, I was able to find a toilet flange that installs INSIDE 3" schedule 30 drain pipe. Many of the plumbing supply places I called in metro Detroit said there was no such thing, others heard of them but didn't carry any, although they get lots of calls for such a flange.
I found a flange made expressly for this purpose by J-Tech Prodcuts Co. They also have one for 4" pipes. If anyone is interested, here is a link. They will UPS units to consumers.
http://www.jtecprod.com/MH350450.PDF
I'll get mine next week, and I'll let you know how it works.