I'm familiar with the Alternativa3D engine To me it's quite good, but I just wondered about another 3d engine which is able to compete with it. An engine must correspond to the following requirements:
must have a good Stage3D support
be open-source if possible (Alternativa3D is not open-source, it's the major disadvantage)
must have an adequate documentation and tutorials
There are a few good 3D engines for actionscript. Although I only know of these two that support Stage3D at the moment:
Hope this helps.
You may have to compromise somewhat because Stage3D is a new resource so support and uptake especially in free open source engines is going to be scarce. You may need to be more realistic with your constraints and settle for an engine which is not open source or isn't documented as much as you would like.