Joe Maley (Jack Easy) described one of the "split" concertinas which he used in his stage act. At a suitable crescendo in a tune, a slider arrangement in the middle of the bellows would allow the two parts to separate. Weights in the slider end brought in enough air for playing to continue for a few seconds.

The tri-concertina sounds like a completely different beast - more like hen's teeth for rarity - possibly unique. Likely to have been a showman's instrument, I can see it being used in the sort of "Octopus Reel" arrangement that the Dubliners used to do, where each member would blow/bow/strum/pick an instrument while another member would do the appropriate fingering. I seem to remember the McCalmans had a similar trick which did involve a (normal) concertina, mandolin and guitar.

Ross