RAW, the feature only states that you can cast it without a spell slot, not that you always have it prepared, so RAW, you would need to have it in your spellbook, and prepare it from there, with the spell counting against your number of prepared spells (or otherwise gain it somehow, through multiclassing most likely). Then again, Eldritch Invocations use similar language, even for spells that aren't even necessarily on the Warlock spell list (Mage Armor specifically; Armor of Shadows would be very not useful RAW), and I'm pretty sure Warlocks are meant to be able to cast the spell granted by the invocation, so I'm wondering if that's actually RAI.

I'd say the Third Eye feature allows casting the spell even if it's not prepared, and it falls under the "Special Abilities" section of the following rules (EDIT: you can't use the feature again until you finish a Short or Long Rest.)

Casting without Slots

There are several ways to cast a spell without expending a spell slot:

Cantrips. A cantrip is cast without a spell slot.

Rituals. Certain spells have the Ritual tag in the Casting Time entry. Such a spell can be cast following the normal rules for spellcasting, or it can be cast as a Ritual. The Ritual version of a spell takes 10 minutes longer to cast than normal, but it doesn’t expend a spell slot. To cast a spell as a Ritual, a spellcaster must have it prepared.

Special Abilities. Some characters and monsters have special abilities that allow them to cast specific spells without a spell slot. This casting is usually limited in another way, such as being able to cast the spell a limited number of times per day.

Magic Items. Spell Scrolls and some other magic items contain spells that can be cast without a spell slot. The description of such an item specifies how many times a spell can be cast from it.

Before you can cast a spell, you must have the spell prepared in your mind or have access to the spell from a magic item, such as a Spell Scroll. Your features specify which spells you have access to, if any; whether you always have certain spells prepared; and whether you can change the list of spells you have prepared.

Problem is this section. It doesn't say you need to have a spell prepared to cast it with a spell slot, it says you need to have a spell prepared to cast it at all (unless you have an appropriate magic item, but Third Eye isn't an item).

Specific overrides general; the feature is telling you that with it you can cast the spell, and when you do it doesn’t use a spell slot.

Quote from artemislena >>

Before you can cast a spell, you must have the spell prepared in your mind or have access to the spell from a magic item, such as a Spell Scroll. Your features specify which spells you have access to, if any; whether you always have certain spells prepared; and whether you can change the list of spells you have prepared.

Problem is this section. It doesn't say you need to have a spell prepared to cast it with a spell slot, it says you need to have a spell prepared to cast it at all (unless you have an appropriate magic item, but Third Eye isn't an item).

 But it says: "Your features specify which spells you have access to, if any"

Quote from The_Ace_of_Rogues >>

Specific overrides general; the feature is telling you that with it you can cast the spell, and when you do it doesn’t use a spell slot.

 Ah, yeah, that makes sense.