Can I root locked Droid 3 and flash Cyanogen Before I purchase a Droid 3 from hong kong , I would like to know if I can apply the recently discovered root exploit on a locked phone that i can purchase there.

Pardon my ignorance in this locked phone area because I have rooted  phones here in india but all were available as unlocked devices.
As far as I am know, CyanogenMod is not yet available for the Droid 3. While there maybe someone working on bringing it to that device, it currently is not "supported". You can look at the Wiki page, I think it is one of the more updated pages for supported devices, but then again, it also hasn't been updated since January.

The "most accurate" sites to see if a device is supported is the CM-Nightly Changelog site and the Download site. 

The nightly changelog site may show devices, but that doesn't mean they actually have a "stable" version. You would have to check the download site to see if there is a stable version. But as I said, I don't see the Droid 3 on there right now. I believe the "codename" for the Droid 3 is the "Targa"

I do want to add, that if you know of a way to root then rooting the device should be possible, you just wont be able to flash CM7 (for now).