Seems to affect 2002-2004 3.0L V6 MPV's, typically the rear bank of cylinders. The initial designs of the coil seemed to not disperse moisture properly and over time would lead to a cylinder misfire. There's been a few revisions to fix it, from reading elsewhere it is the new coils with an AC at the end of the part number that are the good ones.
You may never need to replace the remaining 3, but there's a chance that the ones replaced could fail again if they aren't the "good" revision parts.
Typically, they need to take the top part of the engine off to get better access to the rear bank, so if they've already got that off, it might be cost beneficial to replace all (and the plugs). Of course, your 2004 *should* be under warranty and this repair should be covered. They will only replace those that fail when under warranty.