You have two options here, dependant on your final outcome.
The first is the option stated above - you render the 3d animation using a 3d package, and use Flash to add some interactivity to it. You could export the animation as an AVI from Blender and import it into Flash (possibly using Premiere to export it to an FLV with chromakey), or use Swift3D to create a vectored up 3d SWF to use directly. Swift imports .3ds files, and last I checked Blender will export to that pretty standard format.
The downsides of that method is that it will remain a flat animation. If thats fine for your purposes, this will be a perfectly good solution. However, if you need runtime 3d (because you've got a user controlled camera or some such), then you're going to need to step into the world of 3d Flash engines as liaaam stated. The aforementioned Papervision3D is the current open source leader (and it's fork, Away3D is similarily powerful), but there are alternatives such as Sandy, which has an old version stashed away which can run in AS2 (for Flash 8 users). Commercial wise, Alternativa blows everything else out of the water, and is free for noncommercial use.
Papervision, Away and Sandy all import COLLADA standard models (again, Blender exports to this), while Alternativa imports 3ds files. The support for these 3d engines from this forum is going to be close to zero - no one here seems to have any expertise with these things, or if we have we're apparently not sharing our knowledge. These are also very, very code heavy so a good understanding of actionscript will be required, as well as a head for programming.
