Josh Harrison can play almost any position. He's proven to be a solid 3B option. I don't think he'll come cheap though, he's a really good player who offers tons of versatility.
Seager just signed a 7 year 100M deal two years, I don't see DD trading for another expensive 3B and I think Mariners would want a decent amount for Seager.
Nunez is a horrible fielder and pretty meh. He's an upgrade but I don't care for him.
I think most realistic option is a Plouffe or Peralta, or someone of that level, or calling up Devers. I can't see them paying for these 3B, but they also can't afford to trot this trash out there every game. I'm not saying I want either of those mediocre guys but I just think its most realistic outcome.