1) Invasions by Barbarian tribes
2) Economic troubles and overreliance on slave labor
3) The rise of the Eastern Empire
4) Overexpansion and military overspending
5) Government corruption and political instability
6) The arrival of the Huns and the migration of the Barbarian tribes
7) Christianity and the loss of traditional values
8) Weakening of the Roman legions
Option 3, Zeus, Poseidon, and Hades,
Zeus being the king of the gods, and the god of lightning. Poseidon is the god of the sea, and Hades is the god of the underworld.