In "The Seafarer," the poet names “Fate’s three threats." What are they?

The Seafarer, found in the Exeter Book and probably composed around 725 CE, is about the seafarer's voluntary exile on the sea, his sufferings, a belief in a Christian God, and his sense of loss when he considers the pre-Christian world:

All that old life has fled/the celebrations are done--/the weak people now reside/and control the world. . . .Glory has flown,/the nobles of this life/grow old and dry. . . . (ll. 86a-89b)

In this section of the poem, exhibiting the ubi sunt (Latin for where is) motif, the seafarer laments the passing of the older, pagan, world, and his reflection on this loss is often seen as conflicting with the explicitly Christian sentiments that close the poem.  When he speaks of "weak people," he refers to those who are not warriors, those who now plow the earth and are not part of the pre-Christian warrior society that has passed away.

A few lines earlier, just after the seafarer refers to "the joys of the Lord" (l. 65a), he again reverts to a pagan belief system in which fate, not God, determines the length of man's life:

Always and invariably,/one of three things/will create uncertainty/before [man's] fated hour:/disease, or old age,/or the sword's hatred/will tear out life/from those doomed to die. (ll. 68a-71b)

The important words here are uncertainty and warfare, which sum up the Anglo-Saxon view of life: death may come before one's fated time by sickness, age, or warfare, and there is no way, in a pagan belief system, to prepare for that end.  Warfare is included here because warfare is not only a constant element in Anglo-Saxon (Scandinavian) life but also, after disease, the most common way to die.   Life in Anglo-Saxon times was, as Thomas Hobbes said about life in the 17thC., "nasty, brutish, and short."  Even those who are tempted to see The Seafarer as primarily a Christian poem acknowledge that lines like these--which refer to the role of fate and doom--hark back to the seafarer's pagan belief system.  More important, though, is the seafarer's articulation of life (and death) in the Anglo-Saxon world, which reduces itself to death by sickness, age, and warfare.    

The seafarer certainly ends the poem with his belief in a Christian afterlife, but his firm belief in, and many references to, the power of fate is a reflection of his pre-Christian world, a very common motif in Anglo-Saxon poetry, including its greatest poem, Beowulf.