platinum halo 3 nameplate

Games Halo: The Master Chief Collection
1

i completed halo 3 in under three hours with my friends today and they all got the platinum nameplate but i didn’t. The account i used was Manmadeplayer15

2

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> i completed halo 3 in under three hours with my friends today and they all got the platinum nameplate but i didn’t. The account i used was Manmadeplayer15

So you may be experiencing the same issue as what happens with the Monopolized achievement. What happens is the game wants to register the best time between your solo and coop but since you dont have a solo time to compare it acts like you dont have a time, beat it solo, regardless of time, and see if that fixes it.