I hear what is called 'the Qur'aaniyyoon' and they perform three prayers only: Fajr, Maghrib and 'Ishaa'. Are these prayers valid?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Whoever denies one of the five daily prayers he has indeed committed major disbelief and goes out of the fold of Islam as he has denied something that is confirmed by evidence [from the Book of Allaah or the Sunnah of the Prophet ] and which no one can deny.

Allaah Knows best.