Rig Veda mentions purandara - पुरन्दर (one who destroyed cities). Rig Veda praises both Agni and Indra as purandara - पुरन्दर.
In praise of Agni: Rig Veda VI.16.14
तमु त्वा दध्यङ्ङृषिः पुत्र ईधे अथर्वणः। वृत्रहणं पुरन्दरम् ॥१४॥
tamu tvā dadhyaṃṃ ṛṣiḥ putra īdhe atharvaṇaḥ vṛtrahaṇaṃ
In praise of Indra, Rig Veda II.20.7
स वृ॑त्र॒हेन्द्र॑: कृ॒ष्णयो॑नीः पुरंद॒रो दासी॑रैरय॒द्वि । अज॑नय॒न्मन॑वे॒ क्षाम॒पश्च॑ स॒त्रा शंसं॒ यज॑मानस्य तूतोत् ॥७॥
sa vṛtrahendraḥ kṛṣṇayonīḥ purandaro dāsīrairayad vi | ajanayan manave kṣāmapaśca satrā śaṃsaṃ yajamānasya tūtot ||
If we closely observe, both Indra and Agni were praised as eliminator of Vritra and destroyer of cities.
That is the beauty of above mantras, composers of which were different sages, of Rig Veda. They are referring to the same Almighty or BRAHMAN, because Agni, Indra are epithets used to indicate same brahman (RV I.164.46 - ekam sad ...)
Coming to the destruction of 3 cities by Indra, as mentioned in Bṛhaddevatā of Sage Shaunaka, we can infer from the above that Sage Shaunaka explained the deeds of Indra as mentioned in Rig Veda, in an anecdote .
Destruction of 3 cities or Tripuras by Rudra, as mentioned in Krishna Yajurveda 6.2.3, might have followed the anecdote of Bṛhaddevatā of Sage Shaunaka.
One interesting aspect is that Vishnu is also called purandara in Vishnu Sahasranama No.335
The term puram is also used to refer to the three Sarirasthree Sariras viz. the sthula,sukshma, and karaaa sariras. He is purandara because he has torn apart these three and is the Atma beyond these three Sariras.