The Oath of Vengeance paladin gains the Channel Divinity option "Vow of Enmity" at 3rd level (PHB, p. 88):
As a bonus action, you can utter a vow of enmity against a creature you can see within 10 feet of you. You gain advantage on attack rolls against the creature for 1 minute or until it drops to 0 hit points or falls unconscious.
Part of the description of the Elven Accuracy feat (Xanathar's Guide to Everything, p. 74) states:
Whenever you have advantage on an attack roll using Dexterity, Intelligence, Wisdom, or Charisma, you can reroll one of the dice once.
The Hexblade warlock patron grants the Hexblade's Curse feature at 1st level (Xanathar's Guide to Everything, p. 55), which allows you to mark and curse a creature. One benefit of this curse is:
- Any attack roll you make against the cursed target is a critical hit on a roll of 19 or 20 on the d20.
Combining all of these things, what are my chances of rolling a natural 19/20 if I roll 3d20?