I just learnt about the concept of n back tests. Basically, you are presented tokens from a sequence and your task is to determine whether the current token matches the token N steps back.
This can of course be done in parallel with multiple sequences, called "dual", "triple" (..) N-Back, although I haven't seen any implementation of a triple (k, m, n)-Back with different periods in each sequence.
There is a paper which claims to establish a connection between playing dual n back and fluid intelligence. There is also a wired article summarizing the paper.
Check out the flash game presenting a visual and an audio sequence.