Upgrading all 3 in the same day is a very bad idea. It makes troubleshooting very difficult even if you're a guru on all 3 (OS, BES & Domino), plus the BES is high visibility and you should consider the number of users that will be affected if you have to roll back.
Both Jadey and Aroc described the best approach that should be used.
There are numerous considerations to take note of:
1. Remaining domino servers in the environment - what's their current versions (ODS strructure- R5 uses 41 & R7 uses 43)? Coexistence and interoperability are key here.
2. What anti-SPAM software is currently running on the W2K server
3. Backup software currently running on the W2K server
4. Any agents running on the domino BES server - those may potentially become disabled during the upgrade
5. Administrator client needs to be upgraded to administer the R7 server
6. Is Sametime used? If so the version (server & client) will have to upgraded to match the domino version
7. What about the hardware? Does it support the new platform?
8. What about the device models? Some have limited resources (5800 series) and would experience performance issues with the new BES version because of the extra modules that's pushed down during OTA activity.
You need to map out a clear upgrade plan for the BES and Domino which should include a roll-back strategy if things don't go according to plan. The OS is a bit more straight forward.
The strategy should be OS-->Domino-->BES, and you may have to consider a 2-step process for the domino part (R5->R6->R7).