No, that's not the point at all. If we ARE supposed to be great at dpsing 3 targets, why is it he's still below the other casters? So our best output is supposedly 3 mobs, and as such he is still coming in behind, it just further shows how lacking our dps is.
We're not good at single, and the ONLY way we can keep up requires us to dps off-targets, that's the problem and it's not even keeping up. If your optimum still brings you in behind people, there's a problem, that's the issue.
Generally someone in their optimum situation will come out ahead of those in their average situation, shadow priests unfortunately do not.
For example, most ranged, give them a fight to stand still, optimum performance = beating those that are better on the move (melee). Shadow priest optimum, standing still with 3 targets = Still behind the other ranged, and they require an extra variable to be at optimum. The more variables it takes you to be your Max dps, the more you should do in that particular situation if WoW ever presents you with such an occasion.
In my opinion put in this 3 mob situation, we should come right up behind rogues and warriors, not still be outdone by single target while barely keeping up, OR we should get a single buff and a nerf to multi-dotting, not sit in "Multi-dotting is powerful enough to keep up" that just leaves us behind over-all.
Edit: TBH, how much would it hurt if we topped charts when there were 3 mobs to kill, it's not every fight, it would somewhat be our niche. Not my goal, just a thought.