Please forgive the bump, but I have yet to get an answer to my question.
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I looked around last week, but couldn't find an official Adobe site where the download was still available. As a customer, I'm guessing you could still order a replacement DVD, but also did find it posted on various 3rd party sites, such as here:
Though it's possible that these are special Trial offerings, I'm guessing these were just cross posted at some point from the Adobe Store for 3rd party vendors to sell. You could try your SN with one of these, to confirm whether it works as expected.
P.S. If you want to play it safer security wise (probably smarter), you could open a ticket with Adobe Support, and ask them to post it for you. It might already be posted somewhere in a protected location that they know about.
Ultimately it turns out that one of my co-workers had an old copy of the Flex Builder 3 binary handy making the entire issue moot. It's regrettable that Flex Builder 3 is no longer publicly available but no longer a serious issue for me.