I'm not familiar with the 64 bit drivers but you can be sure the technique is the same.
Download the 64 bit, latest Catalyst driver from ATI. Run the install and allow it to extract to C:\ATI...
Open the .inf with CL in its name in notepad. It's inside the extracted packages\drivers\display folder.
Now in the [ATI.Mfg.NTx86.6.0] section, copy and paste to add this line:
"ATI Radeon HD 3850 AGP" = ati2mtag_RV630, PCI\VEN_1002&DEV_9515
Hit ctrl+s to save it. Close notepad. Open device manager. Right click on the video card. Update driver software. Browse my computer. Let me pick. Have disk. Browse.
Now find the .inf you just edited and saved. If it complains that the driver is unsigned, tell it to install anyway.
After, run setup.exe in extracted folder to get CCC. Tell us how it went. Good luck.
Or you could just download the hotfix version of the driver from ATI lol. This doesn't need manual install and will install all through the setup.exe. The above technique is good for beta releases mostly, involving the 3850, dev id 9515. They never include us in those.