What bit depth per channel is the image you posted? (32, 16, 8)?
On my (64-bit) machine, 32-bit HDR images look wonky, but 16- and 8-bits per channel both look fine. (For HDR imagery, I usually retouch in 16-bit.) I suspect this is because on a 24-bit monitor (8-bits per channel), 32-bits per channel is outside of displayable range.
That looks like 3 photos at almost identical exposures. You really need some variation (even a quarter stop plus/minus seems to work) in exposure to do an HDR merge.
Camera Raw and CS6 both report the source images as 8-bit. The merged photo as 16-bit. My display is set to 32-bit. Plus the merge looks fine if done with the 32-bit version of CS6. Whatever is going wrong is only with HDR Merge in the 64-bit version.
Insanity is hereditary, you get it from your children.
But the readout in MergeToHDR shows the exposures as 0.0, 0.0, and -0.03 EV.
Hmm, have you installed all the ACR updates?
There was a bug a while back where ACR was trying to auto-correct images, despite MergeToHDR telling it not to.