Based on your comments and some Google I think by the 3D acceleration you mean the fancy effects that the old Ubuntu versions used to have, e.g. windows zooming out when you minimise but also the more complicated stuff, such as wobbly windows.
I think this is something else than the actual 3D acceleration, which you have already enabled by installing and activating the proprietary drivers.
In order to do these simple effects, please see the top rated answer in What happened to the Visual Effects tab, in the Appearance window? Briefly put, you can use CompizConfig Settings Manager (CCSM) to enable the "3D" effects. The selection in Appearance has been removed, but you can still edit these things manually.
The fancy animations (wobbly windows, etc) are also in CCSM. Just be careful with changing these settings and playing around, if you change the wrong settings you will break Compiz and have to reset to the defaults, as explained in How do I reset my Unity configuration?
If I misunderstood please post it here.