I've read this: The license purchased will be valid for both Unity 2.6 and all future releases of Unity 3.x.
i've buyied unity 2.6.1 PRO. is this license valid for unity 3 PRO?
I believe if you bought your pro license via the store after the 3.0 preorder feature was put on the store page, then yes, it will be valid.
If you bought your 2.6.1 pro license before that time, then I don't think it will be, but you do qualify for the cheaper upgrade price. Enter your current serial number into the box on the store page to find out.
For further clarification, email sales@unity3d.com