First of all I find it funny that someone who plays the London would call a reply by his opponent boring. There are many ways to get an equal position against the london and just copying the white moves is one of them. The opening is boring and does not set black any real difficulties.
Having said that i do not think that Nc3 fits in a London setup (I am no expert) but in my opinion the knight should go to d2 (with the idea to play Nf3 - e5 and Nd2- f3. Moving the knight to c3 invites active back counterplay based on c5 (at some point) taking on d4 and gaining a central majority.
I come across this a lot lately if I try and play the London System.
1) d4, d5
2) nf3, nf6
3) bf4, bf5
4) nc3, nc6
5) e3, e6
...Knights to move by attacking your developed bishop etc.
I suppose I could spice it up a bit with a gambit or something but I also come across a similar game playing e4, namely, the four-knight's game with bishops to follow.
The two are almost identical. It doesn't bother me if my opponent decides to mirror the opening but for me makes it so boring!
Does anybody know any nice lines to get out of this symmetry mirror BS?