Klaus,
12 players is a much harder problem as the cyclic method I used for 13 players no longer works. I am aware of the solution below for 10 triple matches and 3 double matches, however it only balances the partnerships, so everyone gets to play on the same team as everyone else exactly once.
(A D K) v (H J L)
(A G I) v (B D H)
(C F I) v (D E J)
(A C J) v (D I L)
(A E L) v (B G J)
(B C K) v (D F G)
(C E H) v (F J K)
(B F L) v (H I K)
(B E I) v (C G L)
(A F H) v (E G K)
(A B) v (C D)
(E F) v (G H)
(I J) v (K L)
I haven't checked but there will be some inbalance in the opponents. You could try to move things around, keeping the same brackets above but making different matches, some arrangements will no doubt have better opponent balance than others.
Hope that helps,
Ian.