«ثم للتراخي»
Thumma expresses delay or an interval.
In “جاء زيد ثم عمرو,” Amr arrives after Zayd with a meaningful separation. This distinguishes the three particles: waaw basically combines, faa adds close succession, and thumma adds order with delay.
The required interval is contextual rather than a fixed number of minutes. Time between spoken words, stages of a journey, and changes in legal status are not measured uniformly. The rule must be applied with the nature of the acts in mind.