Here are some comments for the popular 3D engines: Away3D, Sandy, Alternativa and Papervision 3D.

1. I love Away3D because it has the cleanest API and the best
documentation and examples... however,  the triangles that disappear
is a deal breaker for me.

2. Sandy... I downloaded it but I did not like the documentation and
examples so I decided not to waste my time and moved on.

3. Alternativa.  That one is supposed to be ready to go.  I did not
spend much time with it because of the TOS.  If they add a clear cut
free license for indy developers on to their EULA allowing commercial
development I will certainly consider it. Licensing and price is a
huge deal.

4.  Papervision 3D.  I like it a lot.  Similar to Away 3D but I
noticed that the performance was way better (honestly) and it did
triangles do not disappear.  It would be nice if they get it ready to
go soon.  A lot of changes going on. The documentation is outdated and
that can cause some problems.

Conclusion:  For now I am using PaperVision. If Away3D gets things
fixed soon and the performance improves I would switch in a heartbeat.
Of course, if things keep breaking in GreatWhite (PV3D2) I and Away3D
is not fixed I will really get in touch with the Alternativa guys.

Regards,

Koder