According to Tafsir al-Qurtubi, these three days were Thursday, Friday and Saturday. On Sunday, the punishment descended upon them: وَأَخَذَ الَّذِينَ ظَلَمُوا الصَّيْحَةُ (And those who transgressed were caught by the Cry - 67). This awesome Cry was that of the archangel, Sayyidna Jibra'il (علیہ السلام) which was far more terrorizing that the combined thunderbolts of worldly lightening could ever be, something human senses could not take. All hearts were rent apart by the horrific sound resulting in the mass destruction of those people.

From this verse we learn that the people of Sayyidna Salih (علیہ السلام) were destroyed by a severe Sound, but what Surah al-A` raf says about them is: فَأَخَذَتْهُمُ الرَّ‌جْفَةُ (So, the earthquake seized them - 7:78, 91) which, as obvious, tells us that the punishment visiting them was that of the earthquake. Commentator al-Qurtubi has said that there is no contradiction here. It is possible that the earthquake came first and then they all were destroyed by the severe Sound. Allah knows best.