Hi palpman,
I use the Mirrorless Mover 20, which is the next size down for a m4/3 camera, plus two lenses. But I think it will be too small for three Merrills, especially if you need the DP3M inside.
For my DP2M, I use a "large" point and shoot pouch, I can't tell you the name of the pouch right now, it just large enough to hold it, plus 3 batteries and the charger.
Your application might be better served by the Mirrorless Mover 30. Or perhaps three wraps (for each camera) to place into an existing backpack.