From the documentation for SP 6, Hotfix 1:
| Software version |
Before you install the hotfix, verify that the BlackBerry Enterprise Server version 3.6 Service Pack 6 for
Microsoft® Exchange is installed.
1. On the taskbar, click Start > Settings > Control Panel.
2. Click Add/Remove Programs.
3. From the list of currently installed programs, click BlackBerry Enterprise Server.
4. Click Support Information.
5. Verify that the version number is 3.6.6 (Bundle 7).
Original installation media
The hotfix installation file requires access to the installation media that was used to install the BlackBerry
Enterprise Server. Depending on how the software was installed, this might be a product CD or a network
Since it mentions version 3.6.6, bundle 7, as a requirement, I'd have to say you need to step up to SP6.
But SP 6 itself seems to be cumulative. I've not read the entire documentation, but I've not yet found any requirement to install previous service packs first.