Whisk tapioca flour, brown rice flour, white rice flour, sugar, xanthan gum, and 1/2 teaspoon salt together in the bowl of a stand mixer fitted with the whisk attachment.

Cut in butter using the whisk attachment, starting on low speed and working up to medium speed until the mixture resembles coarse crumbs. Stop occasionally to scrape down the sides and bottom of the bowl.

Gradually add milk while the mixer is on low. Gradually increase speed and mix until batter is well combined. Cover the bowl and let the dough sit for 15 minutes.

Preheat the oven to 325°F (165°C) with an oven rack in the center position. Coat 2 sheets of parchment paper with cooking spray.

Place dough on one coated sheet of parchment paper and top with the other coated side facing down. Roll dough to 1/8-inch thickness. Remove the top sheet of parchment and adjust edges for even thickness.

Cut dough into cracker shapes using a pizza cutter and prick each with a fork. Slide parchment paper with crackers onto a baking sheet and place in the oven.

Bake in the preheated oven for 3 minutes. Sprinkle with sea salt, then continue baking until crackers are golden and crisp, about 18 to 25 minutes.