Radiating Gnome
Adventurer
So initially, it's PCs vs NPC Slayers vs The Dragon. Thoughts?
As a personal rule, I try to avoid those sorts of three-way battles unless all sides are focusing on the PCs.
4e combat takes a while to play through -- I enjoy it and all that, but because it takes a long time to play through, the players are not engaged all the time. They're directly engaged with the action at the table when they're attacking or being attacked, for the most part. So, in a typical one-on-one fight, at least one PC is engaged with the action at any given moment.
But if you add a third group to the scene, one that is fighting the same enemy some or all the time, you create chunks of time in which you're essentially playing (with) against yourself while the players watch.
If there's a story-motivated reason to put the NPCs there, I'd recommend a couple of options:
1. Don't play through NPC turns against each other. With a little path up front you can speed that up a lot. (For each NPC, figure out what his or her average damage, multiply that by the NPC's chance to hit, and you get a number you can subtract from the dragon's total HP each time that NPC takes an action against it. )
2. If you can work it out, make the NPCs allies of the PCs, and turn over running the NPCs to the players.
-rg