ABSTRACT Hawking’s area theorem is one of the fundamental laws black holes (BHs), which has been tested at a confidence level $\sim 95~{{\ \rm per\ cent}}$ with gravitational wave (GW) observations by analysing inspiral and ringdown portions GW signals, independently. In this work, we propose to carry out test in new way hierarchical triple merger (i.e. two successive BH mergers occurred sequen...