How can one respond to those who cite Quranic verses and Prophetic hadiths to justify performing only three prayers a day, or combining prayers without a valid excuse?

It is necessary to consider combining and reconciling the proofs, and not delving into matters of Sharia (definition) except for specialists, and not engaging in ijtihad (definition) when there is an agreed-upon text. The prayers are five, and their times are five, and the Sunnah (definition) has accurately specified them. The five prayers are not combined into three times except in cases explicitly exempted by the Sharia, which are: travel, combining for the pilgrim in Arafat, Muzdalifah, and Mina, combining due to rain, due to fear, due to illness, and due to need and hardship. As for combining without any excuse, there is no evidence for it, nor has any reputable scholar stated it. The hadith (definition) of Ibn Abbas is an argument against those who use it to justify absolute combining, as it is specific to a need. As for the verses, they contain generalities that the purified Sunnah has clarified.