It says under the description for Distant Strike:
At 11th level, you gain the ability to pass between the planes in a blink of an eye. When you use the Attack action, you can teleport up to 10 feet before each attack to an unoccupied space you can see.
If you attack at least two different creatures with the action, you
can make one additional attack with it against a third creature.
Would the only way to attack two different creatures be Two-Weapon Fighting?
Best Answer
Distant Strike is for Speed and Damage
By the time a Horizon Walker gains Distant Strike, the character already has Extra Attack, allowing two attacks per Attack action. A character can move between attacks, and there's nothing requiring all your attacks to be directed at a single target:
While focusing attacks is often considered a good idea in action economy terms (dead enemies get fewer turns that injured ones), Distant Strike gives a reason to split them up a bit - extra mobility from those 10' planar jumps - and lets the character deal more damage when doing it. In effect, Distant Strike grants the same damage output a Fighter 11 gets (via their better version of Extra Attack), the Horizon Walker is just forced to spread it around.
Two Weapon Fighting Doesn't Help
Unfortunately, Two Weapon Fighting is not part of the Attack action and does not trigger Distant Strike. If you hit three targets, you're doing the same damage as you would without using your bonus action for a third swing.
Furthermore, using a bonus action for Two Weapon Fighting, prevents using it for Planar Warrior. At L11, Planar Warrior adds 2d8 force damage and makes the entirety of one attack deal force damage. Considering how rare resistance to force damage is (let alone the extra dice), Two-Weapon Fighting is a bad idea for Horizon Walkers.
Advice for More Damage - Get ye a Flame Tongue!
Adding an extra 2d6 fire to each of those three attacks lets you spread a whole lot of damage around. Seeing as it adds to the weapon damage, as I read it, that damage would also become force damage on the one Planar Warrior attack. (Your DM may interpret differently.)