In that case, your writer will be activated when timestep_index % 5 == 0 && timestep_index % 3 == 0, thus for the 0th, 15th, 30th, … timestep (or cycle).
I guess this adds another question – should the Trigger option be Time or Time Step/Cycle?
Currently, we only have timestep/cycle based triggers. However, the trigger infrastructure is extensible. We should be able to support other types of triggers in the future, even Python-based ones.