Marking targets doesn't mean "only damage this target". You can mark a target while still having the DPS dot the unmarked or the (2) and (3) targets or AoE the pack down.

The purpose of marking targets in 4-man dungeons is so that your two DPS don't completely split their non-DoT damage. It simply communicates to the team which mob you intend on building the most aggro on, so they can DPS accordingly. It doesn't say they can't use DoTs or AoE on the other targets.