The situation is a bit more complicated than just 3d acceleration.
You should have 3d acceleration in the present drivers of Trisquel 5.5. You won't see this on the desktop though as everybody (including Trisquel) is in the middle of transitioning to new desktop environments.
That is to say Canonical has taken some radical steps in moving away from GNOME. The Unity desktop environment is Canonical's replacement for GNOME. Canonical has not implemented all the features of GNOME yet.
Most of the distributions based on Ubuntu have decided not to transition to Unity though and are going about creating new or desktop environments or derived tweaked ones based on new and old components. This is a large undertaking and why Trisquel 5.5 took so long to get released.
At the moment what Rubén decided to do in order to gain broader support across hardware was to design the desktop using non-3d fallback components.
Most distributions have compromised certain features (Ubuntu, Linux Mint, Trisquel, and others) although are taking steps which will ultimately result in an improved user experience in the upcoming releases (Trisquel 6, Linux Mint 13, Ubuntu 12, etc).
To conclude you can utilise the 3d acceleration although not with the current Trisquel desktop.
