The problem is likely your CPU. The GPU score is slightly higher than my two 5870's in CF, but I'm running an i7, which scales much higher with multiple GPU's.
Here is a benchmark of the two different CPU's when overclocked to the same frequencies:
http://www.xbitlabs.com/articles/video/display/radeon-hd5870-cpu-scaling_4.html#sect1
Notice how the performance increases a lot more when the GPU is not bottlenecking. With two 5870's, the i7 advantage becomes a lot more apparent. With three 5870's, it's going to be huge. With a faster CPU, you'd probably see higher GPU scores as well as CPU scores.
From the same artical with Vantage scores:
http://www.xbitlabs.com/articles/video/display/radeon-hd5870-cpu-scaling_3.html#sect2
Yours is a good chunk higher than their CF setup, so yours looks about right.