Read it a long time ago, so I know it's really old (back when Dragonball was still running on SJ). Main character is a young boy with three eyes and has a girl companion who's a bit older than him. Forgot what, but they go on some adventures and whatnot.

3x3 Eyes?
The girl is the one with 3 eyes though.

No, I thought it was 3x3 too, but it's not.

The main character is young child and the girl companion is a teenager. I think there was another character too, some old grandpa who helped them out or something...

there's this super old manga by Tezuka Osamu The Three Eyed One
may be too old to fit your description...

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its cold down here fam ~

Maybe Mitsume ga Tooru?

That's what I thought as well.

There is a third eye person in this manga, but the main charater is a young boy with a teenage girl companion but he does have have marks in his forehead (if you got things mixed up).
666 Satan

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If you want to succeed
Then you don't need Good Luck
You need the ability to overcome Bad Luck

Happines and Light needs a source to exist
Hatred fuels itself and Darkness exist where there is no Light


† Lightmare †

Chronicles of the Cursed Sword? the main character has marking on his fore head and he is traveling with a girl.

Lol, nope that's the one. I read a few volumes of it when I was a kid.

Thanks for finding it.

Damn I feel old now though.