You're missing my point.
PRE 1.4, how would this be possible with 3 or more mercs? If that's how the boss was designed in the first place, it is flawed.
Having three Mercenaries in a fight with such a mechanic can simply be concluded as a flawed group composition. I mean, I would never bring four melee DPS to pioneer Hard Mode Jarg and Sorno, especially with everyone still in Columi gear back then.
If you insisted on running three Mercenaries pre 1.4, you would have to counterbalance that with other classes that have shorter interrupts such as Sith Warriors with 8 second interrupts and Powertechs with up to 6 second interrupts with 2/2 Hitman, which will give you ample opportunity to interrupt most Calibrating Shots.