OK, so if your card has 3 wires for monitoring the fan speed, it's totally possible to get it working for HD3870s as you can see from the pic above !!
Go to RegEdit ( backup your registry if you think you'll mess it up .. I take no responsibility for anything you do to your own machines ! ).
Find all instances of following values :-
.. and change the default "1" to "2".
When you've changed all instances of those values ( I think there were 3 or 4 for each in my registry ), close RegEdit -->> Reboot.
When you load CCC up next time, you should see the fan speed slider has appeared and it works too !!
Only one thing seems to be a niggle .. when upping / lowering the fan speed, I have to take it from current value to a higher %age and then up/lower the speed -- if it's say @ 50% and I move the slider to 30%, it still seems to stay @ 50% .. If I move it from say 50% to 100% and then drop it to 30%, the setting sticks.
Hope this is of use to anyone else wanting to mess around with their fan speeds etc ... and AFAIK it works with almost all cards with a 3wire fan connector .. so it might be of use to HD2900 users aswell
Let me know how you get on with it, or if you find a workaround for the slight niggle I mentioned above