You wouldn't be able to duo with them due to the limitations on who you can duo with (within 1 tier for most people, and even tighter restrictions for diamond+ players).
Still, if you duo with someone with a higher elo than you, and you can manage to win those games, it'll boost your MMR more than if you just went solo. Theres no guarantees where you'll end up in the end, and getting placed plat is quite difficult (as after all, plat is where even challengers get placed). If you really had a low diamond MMR at the end of season 6 then if you do well in placements I'd probably expect high gold as your placement. (That said, I'm extremely skeptical of being only gold 3 with low diamond mmr, as you'd essentially need to be dodging matches constantly to keep your LP down, which would be strange to do as a mid gold.)
Still, there might be hope for low plat if you do something like 10/0 placements and queue with someone who has a high plat/diamond mmr, provided you can find someone with that description who is able to queue with you.
