Kind of a mystery to me why Jedd Gyorko hasn't been signed by somebody. Dude had 2000 innings of average D at 3rd over the past 5 years, and, while 2019 was horrible, has been an .800+ OPS guy for 4 out of the past 5 years.
Dougherty on the loud whispers about CK.
https://www.washingtonpost.com/sports/2021/03/19/carter-kieboom-washington-nationals-third-base-options/Neither Castro nor Garcia have gotten playing time at 3d this year. Really the best alternative to CK on the roster is Harrison. Were he to be the regular, then a guy like Mercer or Perez needs to make the 26 man to be the primary backup at 2d/SS/3rd. Castro has not played much 3rd. That'd be a defensive downgrade from Kieboom. Garcia hasn't shown he can hit better than Kieboom for more than a short stretch in August last year and really needs to be kept in the middle infield in the minors rather than rushed.
Gyorko to me seems the obvious fix. Sign him if he's in shape, get him some PAs in West Palm in camp. He probably will not be ready, but Harrison or perhaps Kieboom can play 2 weeks while Gyorko gets ready.