What are the three things that the Messenger of Allah, peace and blessings be upon him, forbade speaking about?

The Messenger of Allah, peace and blessings be upon him, forbade speech concerning many things, among them:

Speaking during prayer, except for its benefit. Speaking about Divine Decree (Qadar). Speaking about the children (al-wildan), as in the Hadith: "The affair of this Ummah will remain close [to righteousness] until they speak about the children and Divine Decree." Al-Manawi interpreted "al-wildan" as the children of the polytheists, or the state of the children of Paradise, or as a euphemism for sodomy.