The basic idea of staged rocketry is to get rid of the mass of empty tanks et cetera as soon as possible during launch, isn't it? So you would ideally want to drop them one at a time as soon as they are emptied.
AFAIK, Delta IV Heavy does, and Falcon Heavy and Angara are planned to, drop their common booster side cores both at once (with Angara planning up to 6 strap on common cores, I don't know if they will drop 2 at a time or all 6 at once). But why not drop just one at a time?
One argument for simultaneous separation might be to maintain symmetry and stability. But at least naively, if the rocket rotates 90 degrees so that the three cores are on top of each other, then removing one of them should not create any asymmetry.
Also, while SpaceX plans to reuse its boosters, Delta IV Heavy does not. SpaceX needs boosters on their side cores in order to soft land them. But why does Delta IV Heavy have rocket engines on its side cores? Engines which are dropped when there's fuel left in the central core. Why not fix all rocket engines to the central core and just use drop tanks on the side?
For Atlas V, Ariane 5, Proton, SLS the empty weight of the smallish side boosters might be too little to care too much about. But it must be very different with rockets using 3 or more similarly sized cores in a bundle.