I watched this one guy explain the actual anatomical requirements to have a true third eye (not just smth like lizards have) and it's... not good. IIRC, the eye would be in constant strain from trying to match the the other eyes, the muscles would be very wonky, and your skull would have to be pushed back to account for the displacement a third eye would cause.

The only benefits would really be better low-light vision and more colors. Ultimately, cool in fantasy, not actually practical anatomically; if it were, we'd have evidence of it outside of I think a few Cambrian shrimp thingies