The third beta carries 25 official fixes over VirtualBox 4.0 Beta 2, including 3D support fixes, proper 3D driver installation when DKMS is not present, fixing VMM performance regressions for 32-bit SMP guests, multi-sector BIOS reading, performance optimizations for storage devices, and other work.
More "performance optimizations" for the VirtualBox storage support is a bit concerning when our Oracle VirtualBox benchmarks from earlier this week against a Linux host and KVM (the Kernel-based Virtual Machine) show VirtualBox not asserting sync/fsync requests thereby already putting your data at risk.
The performance regression fixes elsewhere in VirtualBox 4.0 Beta 3 also don't seem to address the other VirtualBox shortcomings discovered in that article. There's also a matter of its OpenGL acceleration performance.
The Oracle VM VirtualBox 4.0 Beta 3 announcement can be found in the VirtualBox Forums. Based upon the rate they are pushing out these betas, VirtualBox 4.0.0 final should be out in no time.