As Slin said, browser support is the big issue there.
You can safely bet on Flash to work cross-browser and on all platforms, but Flash does not come with 3D functionality out of the box. You'd need to use something like papervision3D ( http://blog.papervision3d.org/ ).
Unity3D recently has ability to export to flash though, and you can create 3D content through that as well (which I think is the easiest way).