Is praying in the ancient mosques of Cairo considered among the "journeying for the sake of worship" (shad ar-rihal) that is forbidden for any mosque other than the three mentioned in the Hadith?

If the travel is for the purpose of praying in ancient mosques and engaging in worship there, this falls under the prohibition of setting out on a journey for other than the three mosques. However, if the ancient mosques are close by, or if the travel is to see them and then pray in them if it coincides with prayer time, there is no harm in that. This is indicated by the Prophet, peace and blessings be upon him, who said: "Journeys are not to be undertaken except to three mosques: Al-Masjid Al-Haram (the Sacred Mosque in Mecca), the Messenger’s Mosque (in Medina), and Al-Masjid Al-Aqsa (in Jerusalem)."