I am using three monitors with Window 7 and Fusion 7 from my rMBP. Two are Display Port, one is HDMI.
Did some searching around here and found a post that detailed the problem with Fusion 6.
From that post, mudaltsov wrote this:
We're gonna work on resolving this problem in Fusion, in the meantime here is a workaround. Shut down the VM and quit Fusion, then add the following lines to the .vmx file:
svga.autodetect = "FALSE"
svga.numDisplays = "3"
svga.vramSize = "134217728"
svga.maxWidth = "8192"
svga.maxHeight = "4096"
This will turn off the automatic detection of the display limits, and allow the VM to use up to 3 displays. You can change it to 4 if you think you might connect a 4th display (using both Thunderbolt ports and HDMI). The absolute maximum number of supported displays is currently 8.
I followed the above steps and it works fine in Fusion 7 too.