Stuck on level 30 (5-6)

[Jonny]

I've got up to this point in level 30.

http://i.imgur.com/qvuRnjl.png

I'm now totally stuck - I've been staring at the top left corner for over an hour trying to figure out what I can do.

As far as I can see, there are 7 possible combinations, (See http://i.imgur.com/6Br2fGj.png ).

I can't see why any of those are invalid - am I supposed to work on a different part of the board?

[Berahlen]

You're supposed to work on another part.

Take another look at the {3} you grayed out above the right cluster.

[RePlay.be]

Damn, was stuck on the exact same spot and made the same mistake of greying that one out. Thanks for the tip Waypoint. :)